The things I'm reading in the USAGE file seem to indicate that it should be able to take a mono file as input without a problem, so I would check to see if there's something about the file that's slightly nonstandard, something that doesn't bother musicmatch but that confuzzles LAME. Things like, the creation program is appending some sort of metadata or tag to the file for example. Like in CoolEdit, there's a "save extra non-audio information" checkbox that you need to uncheck.

Otherwise, try this command line parameter (from the USAGE file):

Quote:
For WAV and AIFF input files, using "-m m" will always produce a
mono .mp3 file from both mono and stereo input.

The only problem is that I'm not certain the empeg will play a mono MP3 file without having something funky go wrong with it. Dunno, don't recall. I remember futzing with this once when I was doing a little project and decided that even though the source files were mono, I'd encode stereo MP3s because it solved certain problems. But now I don't remember why exactly....