Disabling In-game Music

In your game console (~ typically) just type "MP3 STOP" (it's not case sensitive).

You can also add that to your "userconfig.cfg" file although it may not always work. If you don't have a userconfig.cfg file just make one with notepad and put it in the same place as your game config.cfg file. Userconfig.cfg gets automatically executed when you start a game, and is the best place to put custom settings like this.

You can also bind a function key so you can stop the music anytime:

bind F12 "mp3 stop"

(or whatever key you prefer to use).