After I got Rioplay to work, I realized that I have a lot of WMA files. Too bad Rioplay doesn't seem to play them, the shoutcast is pretty cool.

I don't believe there's any open source WMA decoders out there, so as long as that's the case, WMA can't be supported by Rioplay. Sorry

If you're interested in streaming but still want to be able to play your WMA files, have you tried the JReceiver server? It can support Shoutcast streaming with the default Rio client. Check it out at http://jreceiver.sourceforge.net.