The practical philosopher's answer to this apparent conundrum is actually simple. It hinges on the definition of the word sound. If sound is defined as a perceived auditory sensation, then without a perceiver, nothing is perceived, and hence there is no sound because sound is a perception.
But I think most of us would agree that sound refers to the waves of sonic energy emitted whenever something makes noise--a falling tree, a screeching bird, or even the whisper of the wind through the treetops. So, observer or not, the tree does make a sound.
Thought experiment: A man goes on a hike. It starts raining and he is unprepared, so he drops everything and rushes back to his car. A couple of days later he returns to his hiking spot and finds his voice recorder, which has the sound of falling tree on it.