By default, SoundJuicer doesn't give you an option to extract to MP3. You need to set up an MP3 profile to do this, as described here. Note though, when I tried it I had to restart SoundJuicer before Mp3 turned up in the Output Format list (under preferences).
Details : Ubuntu 6.10 (Linux laptop1 2.6.17-11-generic #2 SMP Thu Feb 1 19:52:28 UTC 2007 i686 GNU/Linux)