You might have red the answer of the poster to this volume issue.

He said:

"Thank you for your comment. We are completely aware of all this and everything was calibrated correctly for this video on the product. It boils down to the simple fact that some keyboards just have a sound that is more dynamic than others. Those spikier waveforms stop the overall sound from being quite as loud as other products. This is not necessarily a bad thing as it allows for greater expression as a player."