sound - electricity?
electromagnetic: I'd say, depending on your scale, the best power generation  you're going to get out of sound would be to use a very large and light membrane to catch the sound and tie it to  piezoelectric cells.You want to produce the maximum deflection possible on the piezoelectric cells as this will produce the largest amount of electrical energy. This will probably be cheaper to do by connecting something like a mylar sheet (a cheap place to look would be the foil wrapping-paper at the dollar store) to some piezoelectric cells.I'm unsure if piezoelectric cells alone would receive enough force to produce a noticeable effect.An alternative would be to produce a very large cone and place a piezoelectric cell in the tip of the cone. This would perhaps be a lot less work. However I have a greater understanding of electrics than acoustics, so I'm unsure if it would be better to channel the sound to a small surface or to simply use a large lightweight membrane.Hopefully this helps you some.