It could maybe sound silly or not feasible, but there is already a prototype of a device capable of converting the variations of pressure in the air in usable electric power for small signalling lamps. One example is the design by Hung-Uei Jou,
Enormous improvements to this technology have to be expected to actually be able to use in an efficient way such ideas, of course; nonetheless this could be another way of collecting energy from rather unusual sources and make it available for interesting applications.
On converting sound noise in a playground in energy
What if we were able to store the energy contained in those vital yells that kids constantly emit open-lungs?
