In order to put somethign in-game you have to have music to replace, such as the Tank theme or a jukebox song. You have to make your replacer sound match the replaced sound in frequency, length, and channels, at least.

Luckily, for those of us not in the know, there's a program for that; LuKeM's Sound Mod Creator, which can be found here: http://forum.l4dmaps.com/viewtopic.php?f=19&t=8888
It's pretty simple, so long as you know which sound file you want to replace. The program should do all of the fixing and fiddling for you, and it will easily let you package your sound into a mod so you can share it. I've used it myself, I can vouch for it.

Thank you very much. I will take a look at it!

in both l4d 1 and 2 i have amusic menu which i can play my own mp3s you must encyrt to avoid patent infringement if i can find those links i will post here.