I'm pretty sure that sound effect can be replaced if you put another file with the same name in the same folder. There are only two sound files:
Garrysmod/sound/stealth/alert.wav
Garrysmod/sound/stealth/alertmusic.wav
Put another music file there and see if it replaces the old one. Keep in mind that it has to be ".wav", and it will loop until enemies loose track of you. You can save a file as wav and add loop points with the free tool Wavosaur.

引用自 Max Shadow：

I'm pretty sure that sound effect can be replaced if you put another file with the same name in the same folder. There are only two sound files:
Garrysmod/sound/stealth/alert.wav
Garrysmod/sound/stealth/alertmusic.wav
Put another music file there and see if it replaces the old one. Keep in mind that it has to be ".wav", and it will loop until enemies loose track of you. You can save a file as wav and add loop points with the free tool Wavosaur.

Oh, I didn't remark it , but now, Thanks, I can play like Old Snake likaboss :) . Thanks a lot .
Thanks again for the device , it is very cool !

Well, because it isn't there. All Steam Workshop files are stored inside the Addons folder, compressed in .gma files. However, if you put a file inside your Garrysmod folder, with the same name and path as the one inside the .gma, it will be replaced.

Needs an evasion track for when it goes yellow after a few seconds..that would be epic

Does this mean I have to make a new folder for the sounds?

Make a new folder inside Addons, and put your sound file inside (with the correct path). One of the last updates changed the way stuff is mounted now, so I think you can't put those files inside your garrysmod folder. It has to be mounted as an addon.

I wanna add that we must also convert in WAV 16-bit and not WAV-32 bit as well as in 44100Hz and not 48000Hz to make it work properly.

But thanks for telling us how to change the music by the way.

Either I'm incompetent, or I'm just not doing this right...
I'm failing to get this to work. I've followed both your advice and chopper385's advice and I've come to no luck with getting this changed. So far I have tried making a folder in the addons folder named "sound" leading to "stealth" and then "alertmusic" which was saved in .wav 16-bit with 44100Hz, then tried loading up Garry's Mod only find it didn't work.
Then I tried to take the said "stealth" folder out of "sound" and leave it in addons by itself, this yet again did not work.
Then I tried repeat this same process in the "sound" directory in Garrysmod/garrysmod... Still no luck.
And then I tried to just leave "alertmusic" itself in ths sound directory... None of this is working.

Any thoughts?

If you leave the "sound" folder inside the addons folder, it will take it as an addon named "sound", with only a folder inside named "stealth". That won't replace anything. You need to create a folder inside Addons with any name, and then put "sound" inside it. Also don't try to put stuff inside your main Garrysmod folder. I think one of the latest updates changed the way stuff is mounted, and now the game won't load anything outside of the addons folder.

To make it work and replace the music, i had to create new folders with the original named exactly as the one in .gma, but pictures talk more than simple words, so here is the path with each folder to replace the alertmusic http://prntscr.com/aiph78

Is the number after improved_stealth_mod needed, or is it something specific for your computer or Steam account?

As far as I know, the folder name doesn't matter, but you need to keep the game file structure.

I have one question if its possible, May I make custom soundtracks for the alert tracks and is possible to publish those soundtracks for this mod and give credit?

(Sorry if this question has already been answered)