One has to keep in mind when studying the use of quantum field theory in modeling physics observations that quantum field theory is a mathematical tool that can be used where quantum phenomena appear, not only for fundamental particle physics , as in the standard model that fits the observations of elementary particle interactions.
In the particular acoustic model, they have successfully used a similar to Quantum Electrodynamics (QED) field theory, that is why the gauge exchanged "particle" is called a phonon.
The "equal footing" is in an equal mathematical representation. Photons are part of the SU(3)xSU(2)xU(1) model of particle physics , whereas phonons only model vibrations.
It seems that individual quanta of vibrations have been measured
Now, the same group of researchers has improved their previous design enough to detect individual phonons.
You ask in the comments:
Are phonons' particles similar to other particles we know like photons or electrons?
No, they are particular quantized states of vibrations in solids.
If not, then they are just mathematical entities. Aren't they?
No they are physically observable discrete excitations, as seen in the link above, which are modeled with a mathematical QFT similar to the QFT that models photons so well.
If they are, Do they have all properties that particles have like mass (if massless then they must move with a speed of light which I don't think they do), spin, etc. Why they aren't in the standard model?
Yes, they will have a four vector defined mass, the addition of all the four vectors that make up the excitation. They must have spin 1 to obey a similar QFT to the photon one.
Even the protons and neutrons are not in the particle table of the standard model, which has elementary point particles at its base. Phonons, in an even higher level of compositeness, (they are composite excitations on composite molecules which are composite of protons neutrons and electrons) also are not elementary.