Good free audio converter for OS X Lion?
Get ffmpeg via Homebrew, which can convert a lot of audio/video formats.

Converting your audio files then basically is done by running the following in Terminal

ffmpeg -i input.mp3 -b:a 128k output.mp3


ffmpeg is usually able to deduce input/output format from the file suffix.



To run this over a bunch of files, cd to the top folder containing the audio files and run

find . -type f -print0 | 
    xargs -0 -I '{}' bash -c 'f="{}"; ffmpeg -i "$f" -b:a "${f%.*}.128.mp3"'


This will create files ending in .128.mp3 for all audio files found (and throw a lot of ffmpeg errors for non-audio files in the same folders).