  The Higgs vacuum -- with its constant background field -- is not `empty' but
is a kind of medium. Any ordinary medium, viewed at sufficiently long length
scales, has a hydrodynamic description and can propagate sound waves. The
vacuum medium is unusual; the speed of sound is formally infinite and there is
no linear sound-wave regime. Instead, long-wavelength disturbances are
described by some intrinsically nonlinear hydrodynamic equations.
