Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Aside from checking all possible volume control settings and controls on your Clavinova, I would say you have an electronic problem with your Yamaha piano/keyboard, and it is most likely to be in the audio amplifier stages.
The problem, if it is electronic, will require that the Clavinova either be serviced where it is, or serviced by taking it (!) to a nearby Yamaha Authorized Service facility.
For that to be done (either way) would require that I know where you are located, your local postal code, for instance, or your town/city and province, please.
Let me know what you think, and what you would prefer, please. Thanks.
Have I answered your question enough to rate me positively? if so, please rate my work and my answer (rather than the result... which is disappointing to me, also... I'd like to find a way you could get the problem repaired.)
How old is your Clavinova? is it in good condition?