All online demos these days are recorded into a mixer and the footage loaded into a video editing software program to get the pictures and sound just as they want, unfortunately unless you have the same equipment and knowledge of how to use it, (Not forgetting being prepared to buy the extra equipment) then you will never be able to get your keyboard to sound like the demo. (This is the reason I never make any decision of the sound until I play the instrument myself (Online demos are for entertainment only and serve no useful purpose as to what the keyboard sounds like)

Bill