[lopmotr]

I would call it sound in tennis balls. They're still using the same fundamental mechanism of pressure to transmit sound. But light isn't sound because the mechanism of propagation is fundamentally different, not just at a different scale or with a different medium. It's subject to different laws and behaves qualitatively differently. For example, it can be polarized.

I wonder about a neutron star though? Is that still subject to this theoretical speed of sound limit, or is it only atomic substances?

[ry454]

We can go even deeper and ask how we'd call sound-like gravity waves in spacetime? Some sort of oscillating system of heavy stars can produce a periodic gravity wave that we'd call sound probably.

[jasonwatkinspdx]

No, we don't call that sound. Those are called Gravitational Wave. Admittedly this is a bit confusing vs Gravity Wave, but we appear to be stuck with that one in English now.