I just went through that -- I had some old System 7 sound files I thought would make cool MMS ringtones [Monty Python's "A Message for you, sir" from Holy Grail; a toilet flushing].
I found a program which was free and did the trick, although it was mildly labor instensive:
File Juicer at http://echoone.com/filejuicer/formats/formats?f=sfil