  We introduce an effective point-particle action for generic particles living
in a zero-temperature superfluid. This action describes the motion of the
particles in the medium at equilibrium as well as their couplings to sound
waves and generic fluid flows. While we place the emphasis on elementary
excitations such as phonons and rotons, our formalism applies also to
macroscopic objects such as vortex rings and rigid bodies interacting with
long-wavelength fluid modes. Within our approach, we reproduce phonon decay and
phonon-phonon scattering as predicted using a purely field-theoretic
description of phonons. We also correct classic results by Landau and
Khalatnikov on roton-phonon scattering. Finally, we discuss how phonons and
rotons couple to gravity, and show that the former tend to float while the
latter tend to sink but with rather peculiar trajectories. Our formalism can be
easily extended to include (general) relativistic effects and couplings to
additional matter fields. As such, it can be relevant in contexts as diverse as
neutron star physics and light dark matter detection.
