The non-interacting magnon gas description in ferromagnets breaks down at finite magnon density where momentum-conserving collisions between magnons become important. Observation of the collision-dominated regime, however, has been hampered by the lack of probes to access the energy and lengthscales characteristic of this regime. Here we identify a key signature of the collision-dominated hydrodynamic regime---a magnon sound mode---which governs dynamics at low frequencies and can be detected with recently-introduced spin qubit magnetometers. The magnon sound mode is an excitation of the longitudinal spin component with frequencies below the spin wave continuum in gapped ferromagnets. We also show that, in the presence of exchange interactions with SU(2) symmetry, the ferromagnet hosts an usual hydrodynamic regime that lacks Galilean symmetry at all energy and lengthscales. The hydrodynamic sound mode, if detected, can lead to a new platform to explore hydrodynamic behavior in quantum materials.
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