The poor conversion rates for MP3 encoding you experience comes from the fact that Windows Media Player nowadays by default only enables the "advanced" fraunhofer MP3 encoder, instead of the "professional" encoder. Both these encoders are actually installed with Windows Media Player, however, and the professional encoder may be activated by just tweaking the registry (if you google for it, you'll find several guides on how to do it, I followed the instructions here with good results: http://www.komeil.com/blog/enable-fraunhofer-mp3-l3codecp-acm-windows).

An alternative, at least in theory, is to install and use the LAME mp3 encoder instead. I tried that first, but never got it to work correctly with BIAB.

Good luck!

/Björn