Not sure about batch conversion, but have a look at Audacity.
Features here; http://audacity.sourceforge.net/about/features
Mark
Hi!
I have a lot (100GB+) of audio files (mainly speech) which I would like to convert to MP3, preferably M/S joint stereo around 64kbps.
I was told that the quality depends a lot on the codec (and program) used for conversion, and that some are better for stereo, some for mono, some for joint stereo etc.
Any suggestions for a (preferably free, obviously) batch file converter good for joint stereo?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic