IDK if this counts, but one time when I was mixed, I heard an iPhone jingle/ringtone for 40 minutes straight. I heard it only in my left ear, which was super weird. And the thing is, I'm the only one in my house with an iPhone, and my iPhone battery had died long before I heard the jingle, so it def wasn't my iPhone.

I also heard some techno music in the same mixed episode, but I don't know if I've heard it before or not. It's totally possible I'd heard it before and forgotten about it.

This reminds me of the concept of seeing people in your dreams whom you've "never seen before". You've actually seen them before and your subconscious remembers them, but your conscious does not. So I wonder if the music "that you've never heard before" is actually something you've heard before, but only your subconscious remembers it.