He'd be wrong if he was talking about what physicists talk about when they refer to sound. He'd not be wrong if he was talking about what lots of other people talk about when they refer to "sound".

"Sound" is a word that in our language circumscribes two different categories of phenomena -- the acoustic vibration (that doesn't require a listener), and the qualia of the sense of hearing (that does require a listener). In the circumstances of the English language the two meanings use the same word. That doesn't necessitate for one meaning to be valid and the other meaning to be invalid. They're both valid, they're just different.

If I say "you have the right to bear arms" I mean a different thing with the words 'arms' than if I say "human arms are longer than monkey arms", but that doesn't make one meaning of the words 'arms' wrong and the other right.