My guess is that if it has any effect at all it comes from a metal object vibrating in sympathy with certain musical notes, addin g a sense of high frequency sparkle. There is a company that makes little bells (they call them "resonators") out of precious metals attached to exotic wood blocks. You are supposed to put these in various places around your room to improve imaging. They may very well produce a pleasant effect (never heard a controlled demo, although the manufacturer was going to do one before he got distracted). Maybe a similar effect.
There is no way a couple coins could shift the resonant frequency of a hundred pound cabinet enough to change its damping / ringing properties.
There is no way a couple coins could shift the resonant frequency of a hundred pound cabinet enough to change its damping / ringing properties.