I may be wrong, but AFAIR the usual way to play RM files is to use some binary-release codec from Real. The problem ,of course, is that this means that you can only support RM files on platforms that Real support. There is no easy way to use a binary codec for x86 on ARM(el). The only similar thing, I have seen working is the ndiswrapper for using MS Windows WiFi drivers on Linux, but this is a different case, mostly because the ndis infrastructure is probably documented far better than any codec API.

As I said, I may be wrong, but my it rings a bell somewhere in the dusty parts of my mind... :-)