I have Real Alt 1.51 installed with MPC (I also have QT alt, and it works just fine). I can't play any Real media files from within Firefox nor externally through MPC (MPC says 'Playing' but I have no sound and a black screen). I've checked my Formats settings in MPC; the "hidden" drop down menus for Real media are set to Realmedia not Directshow. I've never had Real Player installed. Anything else I'm missing?

I think sometimes real alternative can not play all types fo real player files. Best to just have RealPlayer.