Question

How can we convert sound energy into Heat energy.

Answer 1

This may help: http://is.gd/mHjh40 & http://is.gd/OzPF1y

Answer 2

Ultrasound waves are able to turn and mechanical wave energy into heat.
So longitudinal waves traveling through vibrating matter and carrying the energy produced in the collision of molecules / particles generating heat. But this applies in tissues and the heat generated is on a small scale.