One solution (though a bit tedious) would be to burn all the music back onto audio CDs (preferably CD-RWs), delete them from the hard drive, and re-rip them at a lower bitrate.
There are also programs that can change bitrate. I think Nero Wave Editor can do it, but it costs money, and does one file at a time. I'd check the Freeware Replacements for Common Commercial Apps
EDIT: Audacity might do it. It seems like the same kind of program as Nero Wave Editor.
Edited by Lanscader, 23 December 2005 - 10:36 AM.