Startup Music
From Valve Developer Community
Source will play sound\ui\gamestartup*.mp3 when it finishes loading the main menu. The asterisk matches anything at all (including nothing), so feel free to give your files descriptive names. If more than one file matches, a choice will be made at random.
All files must be encoded at 44kHz.
Tip:Startup music can be disabled by starting with -nostartupsound.