The exact sound format remains unknown at present. I will attempt to support it in the next release, but it likely will be experimental and unfinished. See more here:
http://wiibrew.org/index.php?title=Wiimote#Speaker
http://www.wiili.org/index.php/Wiimote#Speaker
As far as I know this API (for Java) supports usage of the Wiimote speaker:
http://www.wiili.org/WiiremoteJ