Thankfully you can't
Indeed!!

Some reasons:
Bandwidth - Streaming a full version of a song (4-5 minutes) takes up huge resources and LAGS the game.
Taste In Music - Everyone has different tastes in music. What songs you like, others may detest. I know, I know, it's hard to believe not everyone loves Barry Manalo as much as you do, but the last thing you want to do is drive people away before they even play at your server.
Music Already Playing - Now days, people spend a lot of time on their computers. Because of this, they often have music playing from Cd's, MP3s, Internet radio, etc. on their computer. How annoying it may be when you enter a server and it starts playing music that mixes in with your music? Nothing fun about trying to listen to two tracks at the same time. The quick fix will be to leave your server.
Legal Issues - Do you have the legal right to broadcast the song you want play as background music? Yeah, no RIAA will come see if your server is playing copyrighted music, but it's not nice to do that anyway.
Not giving the right to decide - Can players decide to STOP your music if they want, or pause it, or turn the volume a little bit down? They'll need to do a callvote lol
Extracted from ezinearticles.com/?Why-You-Should-Avoid-Background-Music-on-Your-Website and adapted for Trem servers.