MP3 players, and the DVD library libdvdcss, represent legal problems for US developers. MP3 is patented, and there are *no* licenses available for Linux environments.  libdvdcss runs into the protection of digital rights software in the DMCA, and has even nastier legal protections on it for US developers.

The result is that there are worldwide software repositories such as rpmfusion for MP3, MP4, and a lot of other really useful software, and freshrpms for the libdvdcss that are publicly available. *BUT* you need it to be legal in the country you're downloading *to* to be able to use them without threat of prosecution. There's also the Penguin Liberation Front for a few components with weird licensing that others haven't been able to resolve usable licenses for.

In order to use MP3 legally, I'm assuming you're in a country without that bane of development everywhere, software patents, and can legally use http://rpmfusion.org/. If so, enjoy!!!!