[tsimionescu]

> What if we replace atoms with electromagnetic balls exhibiting similar bouncing properties, would it be good enough to count waves in this medium "sound"?

What do you mean by "electromagnetic balls"?

> My point is that if we can substitute the medium carrying the waves, than we may as well remove the medium from the definition of sound.

Again, sound, simply by definition, is a compressive wave. Compressive waves can only happen in media that can be compressed, which rules out fundamental fields like the EM field or space-time. Atoms may not be entirely fundamental to sound, but matter is - you may be able to have sound waves in a neutron star for example.

I wonder if it's possible to talk about sound waves inside the radius of a black hole - that I'm not sure about.