"Do you hear that?" works as a simple question, asking whether someone can physically hear the guitar or whether they have noticed the sound of the guitar, but it would also require the word "that" to sound correct. I think that would be the most common way to express this idea. ("Do you hear?" without the word "that" would usually be used only if you were wondering whether someone might be deaf in general.)
I think you could also say "Are you hearing that? John is playing the guitar!" and it would sound natural, at least to this American tutor, depending on context. The context in this usage would be an implied sense of doubt, surprise, or disbelief. For example, if John had once been a great guitarist but had refused to play for years, then suddenly started playing. The word "this" is necessary though - you would not say "Are you hearing? John is playing the guitar!"