I am now also using a very dense foam rubber next to the leather pads. It took me a long time to find a kind that does not have a muffler effect as I earlier described here and is durable in the long run.
In the following I describe the changing of leather keypads.
Sometimes a leather keypad becomes soiled with oil a bit or otherwise damaged such that the sound is more disturbing or the seal is no longer perfect. The latter can be checked by removing the key and closing the key hole with some putty such as children's clay. If there is no difference if you are playing d1 and e1 you are fine. If you have to discuss whether there is a difference, try again next day. If it still is not clear, forget about it.
Replacing leather pads is easy if you know how.
I am giving my version, but it is close to Lindsay and Tromlitz . There is one particular detail not found there, it is how to heat the key flap for softening the sealing wax. This is an important detail for doing it without burned fingers and such minor inconveniences.
So, replacing key leather:
-you need sealing wax and a piece of not to thin (e.g. small box cardboard thickness) leather, a piece of wood about 1 cm. thick and a knife or other flat piece of metal. It works easiest if you have some small torch. However it can be done with a gas stove or the like for heating up the knife blade. -take of the key by pulling out the pin. If the pin is a bit stuck have somebody old the foot and hook a plastic ruler or the like behind the small hook that al my key pins have and pull it out that way. Using pliers easily results in damage.
-scrape of the old leather. Do not use force, it is not necessary, you may bend the key.
-cut a slightly larger than flap size piece of leather.
-preferably have the key in a small vise otherwise use a piece of wood and have the flap on it and the end of the key on the table such that the flap is more or less horizontal. Bottom of the flap upward of course.
-heat the sealing wax and drop about 1.5 mm thickness on the bottom of the flap. with sealing wax always use as little heat as possible to do the job. This works easiest with a torch but also a lighter or match will do.
-heat the applied sealing wax on the flap such that it just melts and softly just stick the leather on the molten wax. Again, easiest with a torch but you may apply the heated knife from the bottom. (*) -Now!!!!!! heat a knife blade or such object just so warm that you do not like touching it. Turn the key over and put the leather downward on the wood or the table and apply the warm knife to the topside of the key until the sealing wax just melts. Softly apply some pressure such that the just molten wax comes out on all sides. It should stay more or less of equal thickness. It will be adjusted further later.
-turn the key such that the leather is facing upward and adjust the leather such that it the stem side is flush with the stem side edge of the flap. -turn the key over and rest the leather on the wood. Cut the leather along the flap with a sharp! knife, e.g. a box cutter.
-put the key back on the foot. If the sealing wax now is so thick that the key stands up in a very unusual way, go back to (*).
(**) -If it is more or less in the right direction heat up the knife again like before and warm up the top of the flap such that the wax just starts melting. Softly push the key down with some other object, not your finger, until it looks like the flap is flush with the seat.
-check whether d1 is sonorous. If so ! great, take the key of turn it over with the flap top downward on resting on the wood again and remove the sealing wax that has come out. If d1 does not speak beautifully go back to (**). If it went wrong go back to the beginning, if you cannot manage, send me the foot.
 J.G.Tromlitz, Ausfuehrlicher und gruendlicher Unterricht die Floete zu spielen, Facsimile Frits Knuf Buren 1973, 1985