Convert mp3 to ogg vorbis

On Debian, a quick search with aptitude showed me the packages mp32ogg and dir2ogg. Have a look, maybe they do what you need.

ffmpeg (or more likely the fork avconv if you're using Debian or Ubuntu - these instructions should apply equally to both, though nobody knows how far apart they may drift in the future) should be in the repositories of your distro.

ffmpeg -i input.mp3 -c:a libvorbis -q:a 4 output.ogg

To do a whole directory full of MP3s:

for f in ./*.mp3; do ffmpeg -i "$f" -c:a libvorbis -q:a 4 "${f/%mp3/ogg}"; done

Recursively, with find:

find . -type f -name '*.mp3' -exec bash -c 'ffmpeg -i "$0" -c:a libvorbis -q:a 4 "${0/%mp3/ogg}"' '{}' \;

Set the output quality by adjusting the value of -q:a: for this codec the range is 0-10 and higher gives better quality.

On older versions of ffmpeg, you may need to use -acodec and -aq instead of -c:a and -q:a.

Of course, converting from one lossy format to another is not ideal; but such is life.