There are several things to try. First try closing your explorer window that has the MP3 in it, sometimes having it clicked in Explorer will cause it to not work. You can also try using an mp3 converter, like this one or something.. try converting it into another mp3, that might also help.
One of those should probably do it, if you've tried both and they still fail, try restarting flash and/or your computer and try again.
As said above, try asking this in the flash forum, you will get better help there. Or if it doesn't work you can PM me or something and I can continue to help you out, but for now I'm locking this thread ;P