download ffmpeg, and run this script in each directory:
Code:
for i in *.mp3; do \
ffmpeg -i "$i" -acodec mp3 -ac 2 -ab 192 "${i%mp3}mp3"; \
done
beside the 'ab' is the bitrate you want.
this is just a basic conversion script, so you're going to be left with both the source and destination files - someone with scripting savvy may be able to tweak it so that it recurses and deletes original files, but it's a start.
i would also think that 'sound converter' would accomplish the same thing with a gui...
cheers,