I've noticed that lately startup music wont work unless you edit the autoexec.cfg, so here it is, for anyone else having this issue.
play ui/gamestartup1.mp3
Copy/paste the code in your autoexec.cfg. It should be in your counter-strike source\cstrike\cfg folder. If it doesn't exist, make a new one.
NOTE: This script assumes that your music is in your cstrike\sound\ui directory, it's an mp3 file, and that it's called gamestartup1. If any of this is untrue, edit the file to the correct information.

