Are you tired alt-tabbing so you can play music while playing TF2? This guide will show you how to play music in game without alt-tabbing.

Open the console by pressing the button under the "esc" button. Basiclly, the "~" button.
If it doesn't show up, go to options - advanced... - *click the box next to "enable developer console"*.
If it still doesn't show up, go to Library - Team Fortress 2 - right click and click properties - launch options - and type "-console". It should work now.

Before that, only main menu music are playable, though I will find some ways to play custom musics. Oh, and you can't stop the song unless the server is "sv_cheats 1".
And many typos will come.

Okay, type

"play /ui/gamestartup(insert 1-15).mp3"

Here are the musics:
gamestartup1 = TF2 Main Theme
gamestartup2 = Playing With Danger (Extended class select song)
gamestartup3 = Rocket Jump Waltz
gamestartup4 = The Art Of War
gamestartup5 = Faster Than A Speeding Bullet (Meet the Scout song)
gamestartup6 = Right Behind You (Meet the Spy song)
gamestartup7 = Petite Chou-Flour (Meet the Spy ending)
gamestartup8 = Intruder Alert (Meet the Spy beginning)
gamestartup9 = Drunken Pipe Bomb
gamestartup10=More Gun
gamestartup11=Haunted Fortress 2
gamestartup12=Meet the Medic
gamestartup13=A Little Heart to Heart
gamestartup14=Dreams of Cruelty
gamestartup15=The Calm
gamestartup16=Robots!

It's not finished yet, probably there are some ways to play custom songs. I'll find it.

EDIT: Found a way :D
go to your custom folder, make a folder (doesn't matter what is the name.) and then make a folder named "sound". Then make another folder named "ui". Put all your songs in that folder and voila! If your song is in Youtube, copy the link, go to http://www.youtube-mp3.org and paste it in the text box, press "Convert Video" then "download".

Here's what I found :
-S.N.I.A - Piece A' Piss =http://www.youtube.com/watch?v=5tHUPv7SYdQ
-Daggit Overtime TF2 =http://www.youtube.com/watch?v=8aV9HGFnFnA
-Sentry Beat =http://www.youtube.com/watch?v=n_zvVhnBuQM
-Mmph The Way You Mmph =http://www.youtube.com/watch?v=Mw0re7xmpAQ
-Spy's "Like A Boss" =http://www.youtube.com/watch?v=8Nws_sFMoNY
-Happy Happy Pootis =http://www.youtube.com/watch?v=fnR13Dt8ZR0
-Texas Style =http://www.youtube.com/watch?v=585W28cfz6Y
-Playing With Danger remake=http://www.youtube.com/watch?v=8N3dhccOXX0&gl=ID&hl=id

Did I miss something? Tell me in the comments :D