Dampened piano doesn't work

I'm not sure which "dampened sound" you meant. Certain the standard damper pedal symbol works - it displays in the score and affects playback in the expected way. But if you mean some sort of "prepared piano" technique where special materials are inserted into the piano, there is no indeed no standard playback sound for this in the General MIDI, so you'd need to find a specialty soudnfont that contains the specific sound you are looking for. Obviously the results will differ accord to whether the piano strings are muted with your finger versus a napkin versus a set of car keys, etc.