I was messing around in the sound files a few days ago and found the games startup music, and figured out that you can edit that music, so instead of playing any of valves music on startup you can have tf2 play any song you want.
You just have to go to your custom folder and then UserStuff/sound/ui
UserStuff is whatever folder you have for your hud and such.

Just replace gamstartup1, gamestartup2, etc. with any mp3 you want and it should load any one of those songs on startup.
This is probably worthless I just thought it was cool