I can definetly see your logic here but sound only creates a very small amount of energy.
I think that the real challenge would be to finding a method of turning this energy into electricity on such a small scale - if it can be done then I'm sure that there'd be much wider applications for this technology.
One possible method I could think of would be a very thin material that is highly tensioned at either end, if only the vibrations of that material could effectively turned into electricity
If you could make a self sustainable phone that doesn't have to be charged at all OR which has vastly extened battery life, it would be very marketable. Charging a mobile phone uses very small amounts of power so the environmental benifits would be limited but the convience benifits......................
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I hope I've helped.