The default Ubuntu install includes an application called Sound Juicer for use in encoding cd audio. However by default Sound Juicer cannot encode in MP3 format; this post explains how (at least for Feisty).
First, install this package:
If this package is not found, be sure you have enabled the Multiverse and Restricted software repositories and try again.
Next, after installing the above package, launch Sound Juicer. Click Edit -> Preferences. Next, click on the Edit Profiles button.
Click New. A new window will appear. (Note that on my system none of the fields in this new window can be edited until the Edit Profiles window is closed. So if this is the case for you go back to the Edit Profiles window and click Close.)
Add this text to the following fields:
Profile name: MP3
Profile desc: Encode to mp3 format.
GStreamer pipeline: audio/x-raw-int,rate=44100,channels=2 ! lame name=enc mode=0 vbr=0 bitrate=256 ! id3v2mux
File extension: mp3
This will encode audio files to mp3 format at a constant bitrate of 256; change this number to what suits your needs.
Next, check the Active button, then click Close.
Now back in the Preferences window you should see the MP3 profile in the Output Format dropdown.