First things first, you'll have to find what we'll be replacing. Head to your game directory, which should be located in here: Steam/steamapps/common/UNDER NIGHT In-Birth Exe Late[st]
(You can also right-click the game in your steam library, head to the Local Files category, and select Browse Local Files!)

Now that you're there, you'll see a directory named "d" in there... I'm not kidding, that's the name. Open it, and you'll see that it's filled with mysterious, menacing, extension-less files! The horror! But fear not.
A sizeable amount of them are actually simple .ogg files, which is what we'll be replacing - the game's music.

Through some effort, one can find out that Nanase's Theme, also alternatively known as
"Gallant Girl ~Blowing Wind that Flows from the Sky~" officially, corresponds to a particular file within the directory. And that file is named "zoassNsyootdo12Zm".

Technically, that's all you need to know - do whatever you want with it!
..But it's no fun if you delete it and get only silence in-game, isn't it? How about a replacement?
https://drive.google.com/file/d/1LBiZrsIGR2C3I1T5TU8K6Qx5JCj6uHQr/view?usp=sharing

Take it and paste it into the directory: Nanase's theme shall now be replaced with a different song - May's theme from Guilty Gear XX. The relative lightheartedness of the song is a perfect fit for her, and the sparkly guitar riffs will soothe your ears rather than obliterate them!

You could also do it yourself - in order for your song to work properly, it must be properly converted into the .ogg format, and then renamed to the same gibberish filename. Afterwards, you can paste it into the directory and overwrite the original. Bam! No more will you suffer the cacophony!

I would like to use this opportunity to present my own collection of songs - a good amount of them are actually custom mixes, and collecting/finishing them in total took almost a full week's worth of time: rest assured that at least some effort™️ went into this!

https://drive.google.com/file/d/1TmV47Ohh7_VacjfwCkv942n5ApYCyXlA/view?usp=sharing
Excuse me if I come off as rude, but a good bit of UNIST's tracks felt a bit tame, at least from my perspective as a fan of Guilty Gear's heavy metal aesthetics. This soundtrack reflects that: the style is a bit less glamorous, but I think that it's certainly far more active, and, dare I say, "more hype".

See for yourself - here's a list of all the replacements! Two tracks are unchanged, three are remixed, and the rest are replaced with a selection mainly from Yakuza 0 (I've always thought that it's OST would fit perfectly in a fighting game) and Guilty Gear, alongside some guest stars!
And keep in mind that you don't have to replace everything, if you'd prefer to keep your favorite character's theme, then all that must be done is to avoid overwriting his/her file.

• HYDE | Jukl0wpzoooPgooo0 | -- Pillars of Infinity (from Guilty Gear XX Korean, Kustom Mix)
  A track taken from another sword-wielding, easy-to-use, heroic protagonist. Don't be fooled by the serene harp intro, the determined nature of this guitar will keep the blood pumping! This is actually a combination of both the original Ky theme (Pillars of the Underworld) and Robo-Ky's take on it. (Vortex Infinitum)
  
  
• LINNE | U6oazEogzfbohzk5v | -- You're Mine (from FURI, Kustom Mix)
  Aggressive right from the start, the track combines a creative techno setup with grandiose riffs of heavy metal. The original song is actually my favorite out of FURI's soundtrack, and it was of a purely techno nature. This mix incorporates the addition of a 'supporting guitar', which I think combines the best of both worlds!
  
  
• CARMINE | VwolaulogabioaW30 | -- Bad Surface (from UNIST, Kustom Mix)
  One out of the three remixes. The original flow and speed of the song is preserved almost fully, with the difference being the presence of an extra synth & bass layer going alongside with the original samples. The difference seems subtle on paper, but once one hears this, he can't go back!
  
  
• YUZUHIRA | V9aSgloiawoto0boa | -- With Vengeance (from Yakuza 0)
  This song certainly has a more serious vibe to it in comparison to Yuzu's official theme. It's like a different take on her entire character - graceful, calm, and collected... Now that I think of it, it's like a theme song for her players, rather than the character itself, ha!
  
  
• BYAKUYA | moassNsyootdo13Nk | -- Beat Eat Nest (from UNIST, Unchanged)
  Yep, it's the same. I think it fits quite well with the others as it is: the fast-paced beat and a catchy tune forms the excellent baseline of this song.
  
  
• MIKA | U6oazFodzfbohzk7v | -- Rocket Nuts Groove (from Yakuza 0)
  I think that this one fits the character perfectly: the idle even appears to go in sync with the music! The song maintains the playfulness potrayed in her official theme, but eschews the 'slow' nature of it for a far more active beat.
  
  
• PHONON | PoqDufocuavcouv24 | -- Receive You ~Tech Trance~ (from Yakuza 0)
  Be captured in this elegant track, supported on all sides by techno, guitar, and a soothing (yet subtle) female choir... Look, it's hard to put into words just how nice it is!
  
  
• NANASE | zoassNsyootdo12Zm | -- Water Blue Sky (from Guilty Gear XX)
  I suppose we've already gone over this one - allow me to reinstate that I consider this to be a very solid example on how to do a sparkly, lighthearted theme among a cast of 'edgier' songs.
  
  
• SETH | Zomero0aeceokQpf8 | -- Monochromatic Memory (from UNIST, Kustom Mix)
  The second remix of an official theme on this list! Much like Carmine, the original flow of the song is preserved. This time, the support is mainly granted by... A piano! Turns out that accentuating the notes present in the original song grants it a noticeably more dramatic flair.
  
  
• WALDSTEIN | Y75odToiddleojxzo | -- 怨魔の契り (Vengeful Pledge?) (from Yakuza 0)
  Possibly the most iconic song in Yakuza 0's lineup. Playing as, or going up against a Waldstein now really makes you feel like a true boss is present on the screen with this track blaring!
  
  
• AKATSUKI | Anjf9qfloo6zfobZg | -- Parry Addiction (from Yakuza 0)
  Boxing stance? Check. Agility? Check. Parries? Check. This is an excellent supplement to a battle with Akatsuki, and I think that it fits the electric flow of it all very well!
  
  
• CHAOS | Angf0qfloo6zfobBg | -- Archnemesis (from Yakuza 0)
  Much like Chaos' official, this one differs in style from most of the other tracks. After a hefty build-up, you won't be disappointed by this weighty, epic cocktail with delicious guitar flares!
  
  
• VATISTA | oyqqLqwomrbo07Fkd | -- One Dawn (from Guilty Gear Xrd)
  Akin to Hyde's, this theme features a beautiful and peaceful intro... Just don't get caught off guard by the sudden drop, which leads into an intruging combination of heavy metal and classical instruments with a supporting choir - great for when the match starts to pick up in intensity!
  
  
• GRIM REAPER MAN | PoqUufocuavfouc05 | -- t u s k (from Yakuza 0)
  A powerful song heavily focused on it's guitar, exudes an air of intimidation and confidence - yep, that's Gordeau right there! Not much else to add... ... GRIM REAPAH
  
  
• ENKIDU | Y26odBohddleojkzo | -- Extreme Stream (from UNIST, Unchanged)
  My personal favorite from Under-Night's official soundtrack. It fits both the character and the flow of a match very well. And I really like the traditional wind instruments mixed with the metal!
  
  
• WAGNER | Y16odTodddleojxzo | -- Lily of Steel (from Guilty Gear Xrd, Kustom Mix)
  An extremely active heavy metal song, a great fit for a rushdown character! The mix adds an additional supportive guitar, resulting in an empowering harmony, and also a serene supporting choir & orchestra to give off the feeling of grandiosity present in Wagner's official theme.
  
  
• ORIE | Jukq4wpzoooWzooo0 | -- Firelight (from Yakuza 0)
  Easily the most laid-back and possibly relaxing song in the entire line-up. Fence carefully; follow the ebb and flow as it goes between it's highs and lows... Hey, that actually rhymed well!
  
  
• MERKAVA | Glrohmwo51Rjwotll | -- Unseen Entities (from UNIST, Kustom Mix)
  The third and last remix of an official theme on this list. Like Seth's, the mix introduces a discordant piano, and later on, a guitar, both serving as a supportive layering to the whole song.
  It works, I swear!
  
  
• HILDA | Tyeouzjo10Zadogyy | -- Tor (from Iji, Kustom Mix)
  Iji is a barely known indie game with a kickass soundtrack. Have a taste of it in this theme; perfect to give Hilda the feel of a merciless final boss, with hope and determination shining through in the guitar harmonies of this mix!
  
  
• ELTNUM | Ndos1Qyftoos1soxa | -- Marionette (from Guilty Gear Xrd)
  From one female gunslinger to another - I can't think of a better fit! The dramatic intro gives way to constant crescendos of the guitar riffs with a layer of graceful support: it potrays the combination of deadly beauty well.

That's all of the character themes finished, but as you know, there's a few more...

• HYDE VS. SETH | Lqwomrbo66Rinofco64Codoyqq | -- Two Dragons (from Yakuza 0)
  For a rivals' confrontation, the official theme sure felt like it lacked tension. Hopefully, this fixes that. It's a strong and weighty track, with a beautiful finishing crescendo to top it all off... Let's hope the match lasts long enough to hear it!
  
• GORDEAU VS. CHAOS | osdo0tSod5oayo5et1oOsmNahs | -- The Spider's Thread
  (from Guilty Gear Xrd, Kustom Mix)
  A puppeteer character facing off against someone with great approaches and range? Deja Vu!
  The mix introduces a second guitar for empowered harmonies, akin to Wagner's, in addition to a line of drum & bass. The raising shred iconic to the song will take your breath away!
  
  
• Main Menu | myHoussyomgometo | -- 閻魔の誓い (Exorcise Oath?) (from Yakuza 0)
  Ha, there's 'myhouse' right there in the filename. This is a more dramatic track for our main menu, with the aim of improving one's anticipation toward the matches.
  
• Network | Ewqvjpruhjzjbroovno | -- Interspace Garden (from Yakuza 0)
  A soothing ambient track which will keep you warm on those cold, lonely nights in an empty and desolate player match room...
  
• Character Select | Lqwocicomrbcovomdoyqq | -- Fever ☆ Time (from Yakuza 0)
  It's not as classy, but with it, you'll certainly show some eagerness as you choose your destiny!
  
• Victory Screen | Faskqmoqooskhkwcriw | -- Destiny of the Cats (from Yakuza 0)
  Alternatively known as the "I got beat up again" Screen, the theme displays a greater sense of pompousness from our herculean victor and the struggling yet determined student.

Whew, now that's everything! In conclusion, I'd like to note that these probably fit three-round setups best, else just like the originals they'll be cut off without remorse. Finally, since the game sets a loop point for the music internally, most replacement music cannot be made into a perfect loop, though I've tried my best-- and some do loop quite nicely!


If you'd like to undo any changes to the game files, just right-click the game, head to Local Files and press Verify Integrity - Steam will redownload everything you've overwritten. Alternatively, here's an archive with the original game files:
https://drive.google.com/file/d/1AZEvE6ESjTo2qX_X5wZTSnxgBy8CVZuR/view?usp=sharing

That's all! Tell me what you think, or leave any suggestions that come to mind!

If you fear no man... If you fear no god - then this alternative replacement for Nanase's theme is a perfect fit for you; find out if you can truly survive the onslaught of the dreaded and merciless BIKEHORN: ____________________________________________________
https://drive.google.com/file/d/17J6j4D7NtPP3ucdR2yeZx7sTOpSiE0J0/view?usp=sharing


And here's a more innocent and appealing offering. If you like Gordeau's "Rushing Heart", then here's an mix that adds a spin to it in a style similar to the other remixes up there: ___________
https://drive.google.com/file/d/1mLiGSCrqzBfXeEVDahYJOXuw0c3AQrfH/view?usp=sharing