Fedora aims to be a 100%
"free software" distribution and the MP3 format has a lot of
legal baggage attached to it. If you want to play MP3, you have to do some extra work.
A google search for
"fedora mp3" turns up nearly two million hits, the first couple should give you an idea as to how to get started (it would be wise to narrow that search down for your particular version of Fedora, but you didn't clue us in).
[ June 18, 2007: Message edited by: Joe Ess ]