Existence isn't defined by who is there to sense it. Does a meadow disappear cause nobody is looking at it? It's defined by the question, did it exist, and has anything changed?
Also, sounds are vibrations caused by collision. So essentially, the apple could not have fallen at all in that case. And yet when we go there later, it's on the ground, meaning that it has fallen, meaning that it has collided with the ground, meaning it emitted a sound. If you didn't see or hear it, but you can prove that it happened, it happened.