Yamaha DTXpress/DTXplorer/DTXtreme

Hello all!
Quick question:  Is it possible to play along with an MP3 playing from
my PC (through my dtxpress2's "aux in" function), and record my
drumming (through my pc's microphone jack), WITHOUT having the mp3 be
recorded along with my drumming?

Basically I want to play with a song, but not have the actual song be
recorded.  Just the drums.

Of course I can play my music through my portable cd player and stick
the headphone buds in my ears while I play, but it feels like a poor
alternative.  I figured there must be SOME way to do this.

Any way to mute the "aux in" channel from going through the line out?
 Any help would be appreciated!
-R