Hohner symphonie D98, no key board sound

https://www.ebay.com/b/HOHNER-Vintage-Tremolo-Harmonicas/119095/bn_35515229
https://www.ebay.com/b/Diatonic-Vintage-Harmonicas/119095/bn_25144518
https://www.ebay.co.uk/b/HOHNER-Vintage-Harmonicas/119095/bn_75659859

Normally the model and key is stamped onto the cover plates.

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.

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.

Here is what Hohner says:http://www.hohnerusa.com/index.php?1615 This site is for chromatic harmonicas but the advice applies to all models:http://www.angelfire.com/music/HarpOn/maintclean.html

Try asking by email at this link:http://www.hohner.eu/index.php5?2405 Also have a look at this article at this link:http://www.ehow.com/how_5998550_model-hohner-accordion Hope this helped.

Sounds like you need to flush teh unit out as it has contamination. You might try using mild detergent and DE-IONIZED water.
I would first search google for "cleaning harmonica" for suggestions. You need to clean the reeds and NOT leave any residue. DE_IONIZED water should not promote rust or corrosion if dried quickly after cleaning and rinsing. I would lube the slides with CRC226 available in the electrical dept of Home Depot.

I would be tempted to try spraying CRC226 into the harmonica. It is available at Home Depot in the electrical department, usually near the electrical tape in a blue can about $2.70. It is plastic safe and is a water displacer and a very LIGHT lubricator. If there is crud in there it may dislodge it. It also is good at preventing rust and freeing rusty pliers, etc.

Dis-assemble and inspect for contamination.

Assuming the C-303 has a MIDI out, connect a MIDI cable from the MIDI output of the C-303 to the MIDI input of the symphony module.

Connect two audio cables (mono 1/4 inch) from the symphony module to a suitable audio amplification system OR possibly into auxilary audio input jacks of the C-303, if they exist.

I have a Symphony module but not the piano you have and I have mine connected to a Yamaha organ.