  In sonic models of special relativity, the fact that the sonic medium
violates (ordinary) Lorentz symmetry is apparent to observers external to the
sonic medium but not to a class of observers existing within the medium itself.
We show that the situation is symmetric: internal observers will judge physics
in the external laboratory to violate their own sonic Lorentz symmetries. We
therefore treat all observers on an equal footing such that each is able to
retain a commitment to their own Lorentz symmetries. We then generalize beyond
the case of subsystem-environment decompositions to situations in which there
exist multiple phonon fields, all obeying Lorentz symmetries but with different
invariant speeds. In such cases, we argue that all observers have freedom to
choose which field is symmetry preserving, and so -- in a certain precise sense
-- which other fields are perceived as having an 'ether.' This choice is
influenced -- but not determined -- by a desire for simplicity in the
description of physical laws. Sending information faster than sound serves as a
model of tachyonic signalling to a distant receiver. Immutable causality of the
laboratory setup when perceived externally to a sonic medium manifests
internally through the confinement of the tachyons to an apparent ether (with a
rest frame), which we call a 'tachyonic medium,' thereby preventing tachyonic
exchange from emulating the scenario of a round-trip signal travelling into an
observer's past causal cone. The assignment of sonic-Lorentz-violating effects
to fields that obey 'photonic' Lorentz symmetries thus ensures that causality
associated with the 'sonic' Lorentz symmetries is preserved.
