SoX (it is in synaptic)
I am not sure, but I think it is as easy as:
sox file.ogg file.mp3
If you have a lot of files:
find . -name \*.ogg -print -exec sox {} {}.mp3 \;
it will make file.ogg.mp3 for every file.ogg
(could be that you have to install mp3 support somehow, don't know
about thay.)
--
UbuWu