This script encodes all wave files in the current directory into mp3 files with a constant bitrate of 128kbit. It will not downmix to joint-stereo, but uses real stereo.
Note: Ogg is said to have higher quality than mp3 files, so consider using music.makeOgg instead, which creates ogg files.
(The Ogg format is also Open Source and patent-free, but I'll spare you the annoying "free as in freedom" crap, you'll find everywhere else.)