From TWUUG
#!/bin/bash
# Transcode, copy, and adjust volume of mp3 files for use with
# a small, mono, music-on-hold player. By Mark A. Davis, 2007
#
echo "Warning! About to alter all mp3s in your entire filetree"
echo "from this directory and below!..."
sleep 4
#
for FILE in `find . -iname "*mp3"`
do echo "Starting with "$FILE
# The key with lame is the -V8. The lower the number, the higher the
# quality. 8 is as low as you want to ever go. 5 is much better
# -m m -a is to convert to mono for our music-on-hold system use
lame --noreplaygain --nohist -q 2 -m m -a -V8 "$FILE" "$FILE".hold
# lame will lose ID3 tags when transcoding, use "id3" to copy back
id3 -D "$FILE" "$FILE".hold
# mp3gain will magically alter the volume to seem to be the same for
# all files
mp3gain -r -c "$FILE".hold
mv "$FILE".hold "$FILE"
echo "Done with "$FILE
done