Well, it is possible to mute an H9 with an expression pedal or MIDI depending on how its configured.  The problem occurs without an expression pedal or MIDI connected to it?  Also, is it used?  If so, you should try restoring its system settings by turning it off and then turning it on again with the right footswitch and the HotKnob switch pressed (the switch in the upper left hand corner of the H9).