Disklavier

We hooked our disklavier into our whole house speaker system.  When I am playing just a midi file (which is yes really meant for keyboards so not much actually plays on the piano itself), I can hear the background and keyboard through the speakers in other rooms.  However, when I play a true piano soft album purchased off the Yahama site, I get feedback and no background music through the speakers in other rooms.  The speakers in the piano itself and the piano play fine so as long as someone is in the living room it sounds great.  It just sounds "buzzy" in my family room and dining room.  I thought it might need a ground wire or something, but then when I heard the plain midi files play fine I figured that probably wasn't the issue.  Anyone else experience similar issues?
Thanks
Jean