This probably not as simple as you think. It is not just the right filter, but also the sound will decay faster. This is likely differrent for different frequencies.
If you have guitar at your disposal, you could measure the sound spectum over time when you strike it normally, and once while you dampen it. You can measure the difference in the initial spectrum as well as the difference in decay rate.
You can apply this information to the sound you want to alter, but you'd need to convert the signal to frequency-vs-time first.
But this may be far too complicated for what you had in mind. A simpler approach could be to first increase the decay, by multiplying the signal by e^(w*t), with w as the decay rate. You could split the signal in low and high pass signals and apply different decay rates, with the high freq component getting a faster decay.