Some keys no more sound how to fixed?

Ugh! the only way to fix this is to dismantle your D-20 to get to the bottom of the key bed. There are rubberized strips under the keys that have pads which make contact (hence, key contacts) in several places with the actual keys. The strips are attached with rubber nibs that snap into place on the base of the key bed. The reason for the lack of sound and/or the millisecond delay is a speck of dust that somehow made its way onto the contact for that key. If you decide to take this task on by yourself, make sure you clean the rubber strips (yeah I'd do all of them while you're in there) completely with canned air and contact cleaner in a dust free environment, and be sure to attach the rubber strips completely to make a dust-free barrier. Once you see the strips, the explanation will make sense. If this sounds overwhelming, take the board to a repair shop and their tech will fix this for you, as they see this issue all the time with velocity-sensitive keyboards. Good Luck!

you cannot fix noisy keys...that's how your model is meant to sound

On organ mode, in general, almost all brands of the modern keyboard, velocity setting is not enabled. Setting velocity is used to determine how much the level of the sound that follows how strongly you press the keys. It is really just to imitate the sound of vintage organs, which in the past, electronic organ technologi not use the velocity on the keys. So, how strong / slow we press the keys, the sound level will remain the same.
At the Piano mode,modern keyboard using setting velocity. This also to imitate an acoustic piano, which, if we press the keys slowly, the sound level will be slow as well, and vice versa.
So, in your case,at the same volume slider, try to play on the organ modes, note level. Moving to the piano mode and press the keys securely powered (full power). Then the sound level should be equal to the organ mode.
On modern keyboards, also included organ sound that added velocity setting. Usually found on the user bank.

I have taken keyboards apart and reattached springs that hold keys in position, also use electronics contact spray cleaner to free up the key and switch point that activities the sound

hope this helps?

This could be a multiple of problems depending on what type of keyboard?
I have owned high end Roland keyboards that the soundboard failed, I have an old Casio that some of the keys stick or don't make contact or sound,
and most recently I couldn't get sound on a few different type of midi keyboards for a computer usb hookup,
and to fix that it is in the sound settings and configuration
on the computer, hope that helps?

Without knowing the make and model of the instrument we can't be of much help, although it sounds like a key contact issue. i suggest calling the manufacturer customer service department for help trouble shooting.

Vera hi
to fix this problem you need open where are the contact board keys and clean or change the rubber contact

Sounds like either the contacts are dirty or worn out. Solution clean / replace.

EP-50's are notorious for problems with either the 4.65 voltage feed to the velocity sensitive keys or failure of the small zenner diaodes. Each key has a pair fo split circles on the circuit board under it. When you hit the key you connect each side of the circles and the harder you hit the keys the quicker the circuits close the louder the sound. Problem is if the voltage to either circle fails, due to bad solder, scratch on circuit board, failed diaode then the sound disappears. The fix is simple if you are knowledgeable on solder on circuit boards, can use a multimeter and can dissassemble and reassemble the keyboard. If you have to take it to a repair shop it is probably not worth fixing as this is not one of the better Roland products

I am sure it can be fixed. Who knows what brand or type of instrument this is...

The stuck keys are probably mechanical sticking. The loud keys are probably dirty key contacts that sense velocity.

Take unit in for servicing.