WMA is Windows Media Audio. Microsoft's answer to MP3. Pretty common. Can be played directly on Windows Media Player, WinAmp, or various other players. Can also be converted to other formats via various available tools out there - though conversion from WMA to MP3 may be slightly more lossy than a direct conversion to MP3. What software do you currently listen to your MP3's on?
Another option: Anderson's "Big Science" (CD version, containing "O Superman") is currently on-shelf at the Boulder Public Library. There's no shortage of programs that could convert this to MP3 for you.
