All I can say is I use the built-in sound backend which for N900 is MAFW and for Diablo I think its osso?? So it shares the same backend as the built in media players. In both the N900 and Diablo if you launch the media player with pyradio they will both affect each other.

However you shouldn't need to launch the media player to change sound... you should be able to use the volume rockers because those should raise the volume of the default sound backend.. so I dont really have an answer for that and dont have my N810 readily available for testing this week. Sorry man.