It means that the MP3's have been recorded at 320KBPS, which is the highest and best quality for an MP3. This could apply to any sort of music, but with dubstep when that number starts to decrease, so does the quality and impact of the bass and other parts of the track. The 320 is not a specific reference to dubstep, just how good the MP3 is.
"Yo, can I get a 320 of that sick track?
by Crazy Killalette July 09, 2010