  We show that the commonly accepted statement that sound waves do not
transport mass is only true at linear order. Using effective field theory
techniques, we confirm the result found in [1] for zero-temperature
superfluids, and extend it to the case of solids and ordinary fluids. We show
that, in fact, sound waves do carry mass---in particular, gravitational mass.
This implies that a sound wave not only is affected by gravity but also
generates a tiny gravitational field, an aspect not appreciated thus far. Our
findings are valid for non-relativistic media as well, and could have
intriguing experimental implications.
