MP3 encoders are entwined in legal issues that are too complex to get
into here. Recording your personal CDs to listen to the music you've
already paid for appears reasonable.
The encoder of choice is call lame. The Debian package is available
at http://hive.me.gu.edu.au/.
The issue with MP3 is that the MP3 codec is patented by Fraunhofer in
Germany, and a license is required to use it. To address this a free
audio format has been developed called Ogg Vorbis. There are many
utilities that will rip CDs to Ogg Vorbis format.
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