How do I compress a wma file?

WMA files are already compressed. If you want to e-mail it you'll need to split the file in to small pieces i.e. less than 1Mb, and it will need to be re-embled at the receipients end. If you want to send the file in one hit, you'll have to drop the bit-rate of the audio so that the files is, again, under 1Mb.