misc thoughts:

does the piano have its own volume control by any chance? (not software volume control...I'm talking about a physical volume slider or knob or other control on the piano itself) ...If so it may have been turned down somewhere along the way


Does your piano haver a volume pedal that may have been left at a position that would lower the volume?


Is the Yamaha playing back the midi for other instruments too, or just the piano?
If not, compare the volume settings for each