I read that they were working on media playback on the most recent firmware revisions, not sure how compatible (if at all) the playback capability is at this point.
I've never used my Openbox for playback of anything other than the files created on it, so I don't know the answer to your question. However, if you want a truly compatible, upgradeable, and reliable hardware media playback device, I recommend the Western Digital WDTV Live or Live Plus. These little devices will play back nearly anything you throw at them, are networkable, and are very reasonably priced.