The stupid answer is...
Well, I know it's not what you're looking for such as an ingame command or run or such, but this works for me at least...
1)Make a playlist of your MP3's you want to listen to in your favorite players (I use winamp), turn off all visualizations and run it in a minimum mode if available. Also make sure repeat is on for extended plays
2)Next, run JKII with the music running in the background. Go to your settings and set the music volume to zero
3)Join a game and enjoy it with your choice music in the background
I have a 1000MHz Athlon with 256 megs of sdram and an older model Radeon (original All in Wonder Radeon) and it runs like buttah at 1024x768 with all the gfx settings on almost full (~70-100 fps), and considering that's a midrange system by today's standards, I don't think you'll have much trouble.