Here are some very useful links:

http://www.lmgtfy.com/?q=custom+sounds+source

http://www.lmgtfy.com/?q=custom+sound
Hope that helps

Quote:
For Portal 2 !!
When using custom sound/music in your level, possibly it won't play when activated and you may get this error:
"[Sound] S_StartSound(): Failed to load sound 'destination music/mymusic.wav'. File is missing from disk/repository."

Well now you fix this problem, I give you two options.

First(recommended):
Go to your "\steamapps\common\portal 2\portal2\cfg" and create file called autoexec.cfg then
write this snd_updateaudiocache in the file. Now every time you run Portal 2, it updates audio caches. And that fixes the 'mute' ambient_generic entitys. And why this way? Because when you have custom sounds in your level, you can include this file, when releasing the map, and everyone who plays the level can hear custom sounds.

Second:
Just write snd_updateaudiocache to console and restart map.

Hope this helps everyone who is willing to use custom sounds!

Hope this helps !