Disklavier

My Disklavier Mk III has suddenly gone mute, on all instrumental 
accompaniment playbacks.  If I'm playing a Yamaha PianoSoft Plus 
diskette, the piano still plays fine acoustically  -- but no other 
instrumentation plays at all through the speakers, it just stays 
silent.  I know that the speakers are OK, because I can switch the 
piano to Silent Mode and the synthesized piano voice then plays fine 
through the speakers (but still no other instruments).

Strangely, if I try to play a standard MIDI file, not only is the 
MIDI instrumentation totally silent, but in this case the piano will 
not even play acoustically!

Also, if I try to play just a regular audio CD, which normally would 
work, there is no sound at all from the speakers.

Given that my local dealer has a minimum two-month (!) wait for 
service appointments (which I consider totally unacceptable), can 
anyone suggest any troubleshooting or diagnostic steps that I can 
take in the meantime?

Thanks in advance for any help!


  - Bob