I have this. For me it's like when I'm reading text and I can "hear" what I am reading, or like hearing the sound of my own thoughts. I wouldn't call it a hearing experience, but a somehow different audible thought. And often times it is happening outside of my awareness and when I get a moment to myself, I'll notice a song looping as if it had been for a while without my noticing. I have music playing in my head all of the time. I can trigger it to happen by thinking about a song, or a song can spontaneously take root, or I can replay something I just heard. As a kid this used to happen with things people said. I could "replay" conversations, or sometimes they would replay spontaneously. That doesn't happen much anymore, but it does with music. I always thought it was normal and that this is what the phrase "having a song stuck in your head" refers to.

The air piano keyboard is a neat trick, though. Never done that. hehe.