If you listen to MMS internet radio in Rhythmbox and get a "Couldn't start playback" error, there is a workaround. It could be that your stream is actually RTSP - change the mms: prefix to rtsp.
e.g. mms://wms-trn-all.streaming.net.nz/trn-newstalk-zb-akl -> rtsp://wms-trn-all.streaming.net.nz/trn-newstalk-zb-akl
This is correctly handled by GStreamer but Rhythmbox currently does not handle this. Other applications like Totem and Amarok work fine.