Are we talking about the speed of light here, or the speed of sound? There is quite a difference at those levels, and when we are limiting our discussion to phonons (speed of sound), I think my point stands.
*I* was only talking about this statement: “Mass is not a property that is affected by movement”.
Whether E=MC2 is applicable at varying speeds below that of light is a whole ‘nother matter of discussion. I weigh-in with old uncle Al that, yes, it does apply at the speed of sound as well.
I was talking about mass and speed in general. Once an object theoretically hits the speed of light mass is negated or inverted.