Editing/Adding Startup Music
Music files, however, are in MP3 format at 44100 Hz. They're stored under team fortress 2tfsoundui. Convert them (being a ProTools user, I get QuickTime Pro free - however, you might want to use DBPowerAmp as suggested by the creators of HLSS), and shove them in the folder as gamestartup*.mp3. They may be replaced from the GCF upon startup if they're replacing an actual file (this happens on some Source games, but not on others), or they may not. However, if that happens, you can also add them to the rotation by giving them a number one up from the last one.
Hope this helps