Surely it makes a sound even if no human being is there to hear it.
Humans always think to be too much relevant in the Universe.
... i think that there could be so many living beings near or under the falling tree, may be there are some flowers or some grass down there when the tree falls down.
Perhaps they don't hear a "sound" because they do not have ears, but may be they perceive that a tree is dying.
The tree itself is a living being... may be he (it?) is aware that is falling down.
This is not a quantum physics answer -

Aly