Mp3 is not a proprietary format and there are free software
libraries/programs to play mp3s.
"Many organizations have claimed ownership of patents related to MP3 decoding
or encoding. These claims have led to a number of legal threats and actions
from a variety of sources, resulting in uncertainty about which patents must
be licensed in order to create MP3 products without committing patent
infringement in countries that allow software patents."
http://en.wikipedia.org/wiki/MP3#Licensing_and_patent_issues
OGG Vorbis was created to solve this problem.
