Phonons are not electromagnetic waves, they’re mechanical oscillations (or at least the quantum version of them).
Are you sure there is no confusion between "phonons" and "photons" (either by you or the prof)?
Photons are electromagnetic.
Phonons are elementary oscillations in "elastic fields" (i.e. oscillation in matter). The name was chosen in analogy to photons. As for what they are I am just stealing the definition from [wikipedia](https://en.wikipedia.org/wiki/Phonon
> A phonon is a quantum mechanical description of an elementary vibrational motion in which a lattice of atoms or molecules uniformly oscillates at a single frequency. In classical mechanics this designates a normal mode of vibration. Normal modes are important because any arbitrary lattice vibration can be considered to be a superposition of these elementary vibration modes (cf. Fourier analysis). While normal modes are wave-like phenomena in classical mechanics, phonons have particle-like properties too, in a way related to the wave–particle duality of quantum mechanics.
Phonons are in now way EM waves. If one did want to assign a quantized excitation to the polarization waves that are truly meant when people haphazardly talk about "light in a medium", it would be something like a polariton.
That being said, you sure you didn't mishear "optical phonons" and misunderstood what was meant by that? Optical phonons are gapped phonons, meaning you need a minimum amount of energy to excite them. This is in contrast to acoustic phonons which are gapless. Optical phonons are so named not because they ARE light, but rather that they're the phonon that is typically excited BY light.
I'd err on the side that a professor of chemistry know what a phonon is an you misunderstood the term optical phonon.
Phonons are not quanta of EM radiation.
However there is a common term, *optical phonon*. It means a phonon that can be excited by (i.e. produced by absorption of) a photon in the optical band.
Your chemistry professor is wrong. Phonons are quanta of mechanical excitations, and thus they travel at the speed of sound. There are some analogies with the electromagnetic field, but they are different things.
If I had to guess, he probably confused phonons with photons, which are quanta of the electromagnetic field and *do* travel at the speed of light.