http://media-convert.com/convert/ works great for converting single files- I suppose the only downsides being that A, it's not an app but an online resource and B, you'll just have to open a bunch of tabs to batch convert.
It's a reliable site and I've checked- it can certainly convert .rm to .wav
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http://www.erightsoft.com/SUPER.html
Edit: it's kind of confusing to navigate their page, but what you want to do from the above link is...
scroll waaaay down past all the text and the two windows with example scripts, and click on the "Start Downloading SUPER © right now and discover this fully featured Multimedia Freeware."
On THAT page, scroll down just a little bit to SUPER © is a FREE Multimedia software to download and use and click THAT one.
Now, scroll to the very bottom, and click Download SUPER © setup file . Now you should actually get the installer. You may have to wait a few seconds when the page seems like it's already loaded for this last link to actually appear.
I've e-mailed them asking why a company with such a good product has such a confusing layout, but never got a reply.
It's a reliable site and I've checked- it can certainly convert .rm to .wav