The mass is represented by the density of the material . The wavelength of the sound waves is , and the frequency is . The frequency gets small only when the wavelength gets large. That is precisely the point. Long after phonons were understood, Jeremy Goldstone started to think about broken symmetries and order parameters in the abstract.
Magnets break the rotational invariance of space.
Because they resist twisting the magnetization locally, but don't resist a uniform twist, they have low energy spin wave excitations. Nematic liquid crystals also have low-frequency rotational waves. In superfluids, the broken gauge symmetry leads to a stiffness which results in the superfluidity. Superfluidity and superconductivity really aren't any more amazing than the rigidity of solids. Push on a few atoms on one side, and atoms away atoms will move in lock-step.