Have a go with JetAudio, free from click here
Scroll down to the bottom and select Basic Download.
I use it most of the time and it plays everything I have tried. I don't have any .rm files but they are listed in JetAudio.
Once installed you need to run it and go to Preferences, File Type Association. In the right hand window click on Check All, Update and then OK, twice. It should now be set up to play everything, Audio and Video.
You may need to reboot for the changes to take effect.