Hi @kaitojzy
Try resetting the grand piano back to factory default and check if that resolves the problem.
Turn off the piano if it is on and then press and hold the rightmost white key on the keyboard. Whilst still holding the key, turn on the piano. Once the power is on, release the key. The piano should then initialize back to its default settings.
Don't know the piano but did you check the settings as shown on p.87 of the [https://www.fullcompass.com/common/files/16239-dgx640_en_om_a0.pdf|user guide] or aren't these applicable to the keys?
Looking at the [https://elektrotanya.com/yamaha_dgx-640.pdf/download.html|service manual] the audio amp is common as all the audio comes from the CPU, there doesn't seem to be any separate inputs into the audio amp - see the block diagram on p.8 of 67 for a simplified connection block diagram.