  Sound can exert forces on objects of any material and shape. This has made
the contactless manipulation of objects by intense ultrasound a fascinating
area of research with wide-ranging applications. While much is understood for
acoustic forcing of individual objects, sound-mediated interactions among
multiple objects at close range gives rise to a rich set of structures and
dynamics that are less explored and have been emerging as a frontier for
research. We introduce the basic mechanisms giving rise to sound-mediated
interactions among rigid as well as deformable particles, focusing on the
regime where the particles' size and spacing are much smaller than the sound
wavelength. The interplay of secondary acoustic scattering, Bjerknes forces,
and micro-streaming is discussed and the role of particle shape is highlighted.
Furthermore, we present recent advances in characterizing non-conservative and
non-pairwise additive contributions to the particle interactions, along with
instabilities and active fluctuations. These excitations emerge at sufficiently
strong sound energy density and can act as an effective temperature in
otherwise athermal systems.
