Why isn't sound being used an energy source?

Sound is a product of frequency and frequencies are the primary force behind all tesla and related.

Seems like it would be a free clean source of energy

Probably because you have to use energy to make it. Not to mention you also need air or water to transport sound.

Does not seem practical.

What about the sound of a waterfall
no energy needed

Sonoluminescence can occur when a sound wave of sufficient intensity induces a gaseous cavity within a liquid to collapse quickly. This cavity may take the form of a pre-existing bubble, or may be generated through a process known as cavitation. Sonoluminescence in the laboratory can be made to be stable, so that a single bubble will expand and collapse over and over again in a periodic fashion, emitting a burst of light each time it collapses. For this to occur, a standing acoustic wave is set up within a liquid, and the bubble will sit at a pressure anti-node of the standing wave. The frequencies of resonance depend on the shape and size of the container in which the bubble is contained.

Some facts about sonoluminescence:
The light flashes from the bubbles are extremely short—between 35 and a few hundred picoseconds long—with peak intensities of the order of 1–10 mW.

[link to en.wikipedia.org]

you should also look up the piezoelectric effect. microphones do actually generate their own current this way.

Where did you hear that?