I have 12 CD's of French Language instruction that I stupidly ripped to ogg instead of mp3 (thinking it was somehow better for my karma). Of course, when I later went to put them on my iPod, I was out of luck (and didn't really want to risk putting
ipodlinux on a newer video iPod).
Once again, ffmpeg to the rescue with some help from find. This command will find all ogg files, starting with the current directory, and convert them to mp3. The {} and \; syntax of find takes a little getting used to.
find . -iname "*ogg" -exec ffmpeg -i "{}" "{}.mp3" \;