Well you learn something new every day. I've never heard of that codec before, but after some quick googles on it I now know that it is the best audio codec for voice at low bit-rate. I now also know that aparently older versions of WMP for various versions of Mac OS had some sort of support for it, but from what I can tell there is no such thing as acelp.net for WMP9 on OSX. In fact WMP9 for OSX seems a bit shabby.

Sorry I couldn't be of any more help. Hopefully someone else has a solution!!