MP3 software, similar to DVD players, has certain licensing issues:

• http://en.wikipedia.org/wiki/MP3#Licensing_and_patent_issues
• http://www.mp3licensing.com/help/index.html#5
• http://lame.sourceforge.net/tech-FAQ.txt      (section 6)

Very interesting information here from the Audacity Team:

• The patent holders have tended to enforce licence fees against commercial rather than free MP3 encoders
• Thomson themselves have said that no license is needed by individuals creating music libraries of MP3 files for personal use (interpretations vary whether that sanctions using unlicensed encoders, free or otherwise)
• MP3 patents will expire worldwide between 2010 and 2012

A binary-only, legally-licensed MP3 codec is available here for free.

See similar article on DVD software.