When I'm in a similar situation (unable to compress a file further for sending by email) I usually split the file into two or more parts. The advantage of doing so in this case is that the original quality of the mp3 is kept, but it will require some additional work on the person receiving the file to combine the parts together. This might be done by executing a batch file created by the file splitting program, or using the file splitting program itself to rejoin the parts. (Or perhaps do it through the Copy /b command)
HJSplit (http://www.freebyte.com/hjsplit/) is one program that might do the job. The homepage says that the Java version'll work on the Mac, but I haven't tried it out yet... (I do my splitting on a PC)