I suppose, you can install Audacity for Fedora Core 5 from
here
Find out the settings of volume control, in case you are using KDE. They tend to be mute or pretty low, and you have to keep them to the highest level to ensure, you listen to what you speak over the microphone.
Though you cannot actually record in mp3 format, you can export your recording to a mp3(Lame) format, by installing a dll that would help you do the same