It seems to me that the question is highly ambiguous, because the word 'sound' has two very different but common definitions; the definition you use affects the answer you get.
from dictionary.com :
sound:
1. the sensation produced by stimulation of the organs of hearing by vibrations transmitted through the air or other medium.
2. mechanical vibrations transmitted through an elastic medium...
So Sisyphus, you and Klaynos are using the second definition, aren't you, which means that the tree hitting the ground causes vibrations in a medium, and that this happens irrespective of observers ? So the answer is 'yes, it does'.
Whereas if one chooses to use the first definition, that sound is an sensation, (requiring the existence of an organism using sense organs to experience the sensation), one can truthfully say that the falling tree does not cause a sound, because the question specifies that there are no auditory sense-organs (I could have said 'ears') within range (earshot) of the physical waves, and therefore no sensations.
So yes are and no are both correct, depending on what you mean by 'sound'.
Does that make sense ?