Custom Startup Music

all you have to do is go to...
...\Steam\steamapps\USERNAME\team fortress 2\tf\sound\ui
there are 3 files called
startupmusic1
startupmusic2
startupmisic3
those startup tracks can't be overwritten.
also there are 3 (maybe more) tracks located elsewhere that you can overwrite by naming your files startupmusic4/5/6.

now...to add your own songs, all you have to do is copy an mp3 file and name it startupmusic#.
 if you want to keep original tf2 music, start naming them from 10 to be safe.
they will all be played randomly.
you can add as many songs as you want.

PS: there are some mp3 files that don't work. have not figuret out why but...ohh well...