I hadn't seen this before, but it looks like the Xiph.org folks (of Ogg Vorbis fame) have a codec called Speex that's optimized for voice recordings. The things I encoded sounded very good -- nearly perfect for speech-only audio at vbr, which came out to just below 30kbps, and not bad at all at lower bitrates.

Getting this implemented on the empeg would probably be a very good thing for audio book fans.