Try going into the OS 9 control panel that has "File" at the start of it that isn't "File Sharing" or "File Synchronization". As I recall (I'm working off of OS X, which has an incomplete 9 system folder, aka classic), in that control panel, there's a list of every single file suffix, and which program will answer to each file. I assume you tried opening them from within the MP3 program, so perhaps this is the problem. In that control panel, find the file suffix mp3, and specify your MP3 player to open and recognize those files. I think this will work, though I'm not sure.