This is a very simple Tutorial thats will teach you how to make your own startup music in no time!
First you must download this file http://22.214.171.124/~fpsbgamm/sounds/71/counter-strikegunsounds.rar (this will make any music file you want work with your game)
next drag the cfg file that you got from the downloaded file above into C:\Program Files\Steam\steamapps\USERNAME\counter-strike source\cstrike
now find a track that you want (lets say M.I.A. by Avenged Sevenfold)
Next go into C:\Program Files\Steam\steamapps\USERNAME\counter-strike source\cstrike\sound\media
Drag the file into media (If you find a file in your media file called gamstartup delete it)
Now drag your selected track to your media file
After that rename it "gamestartup" (without quotations) (Also See screenshot A)
Finally after all this is done Close out of the files and Start Counter-Strike
NOTE: FILES MUST BE MP3 FORMAT NOTHING ELSE WILL WORK!