.RM is an abbreviation of Real Media. RM files may contain audio and video. Apart from that, audio is also streamed in RM format.
Open .RM files
RM files can be opened in its official Real Player to be played. However, Real Player is slow and bloated, and may slow down your computer. A better option would be to use the free and open-source VLC player, which can also open Real Media (.RM) files. VLC player can also open most formats other than .RM.