The sound of DHOST
  We show that, in generic higher-order scalar-tensor theories which avoid the
Ostrogradsky instability, the presence of a scalar field significantly modifies
the propagation of matter perturbations, even in weakly curved backgrounds.
This affects notably the speed of sound in the atmosphere of the Earth. It can
also generate instabilities in homogeneous media. We use this to constrain the
viable higher-order scalar-tensor models.
