I have yet to use it, but I stumbled across this yesterday at versiontracker.com. It mentions converting between WMA and MP3.
"Product Description:
Audio Hijack Pro is an audio grabber and enricher, enabling users to record and save any audio. Record Internet streams, capture DVD audio, or convert between any audio format (Real, WMP, AAC or any other) to the more universal formats of AIFF and MP3.
AH Pro can also be used to record, tweak and improve the quality of any application's audio with included DSP effects, as well as industry-standard VST and AudioUnit plugins. MP3 recording, enhanced Timers, and the ability to hijack running applications are just the beginning for Audio Hijack Pro. It also includes the ability to pause recordings, the power to create sets of DSP effects called Patches, support for up to 100 VST and/or AudioUnits plugins at a time, and more. AH Pro even supports some Classic VST plugins which haven't been updated for OS X!