Install the following packages:

sudo apt-get install gstreamer0.10-pitfdll gstreamer0.10-ffmpeg gstreamer0.10-plugins-bad gstreamer0.10-plugins-bad-multiverse gstreamer0.10-plugins-ugly gstreamer0.10-plugins-ugly-multiverse

Set up Sound Juicer:

Open up Sound Juicer-->Edit-->Preferences-->Output Format-->CD Quality, MP3 (MP3 audio)