I SWORE I've played on maps before that had custom round start and win/lose sounds, I've done a search and found posts about adding custom sounds but not about changing the default ones.

Am I just going insane?

im sure changing sounds is like changing skins. you just need to place it in the proper folder with the same name of the audio file you are replacing.
there is no start round music, and i am pretty sure thats a feature on some server side mods

Okay, I'll look more into it. I still haven't found much info thou.