Does the tree make a sound? That's a really tough question to answer. What we hear is filtered through the faculties of the mind so that we may understand sounds that are caused by physical forces outside such as trees falling. So it depends on how you define "sound". Is it the mental quality you hear in your subjective consciousness such as "Crack! Booom!" Or is it the sound waves that we can measure objectively with scientific means? The former sound can't be heard without an observer, (unless the ultimate observer, God, senses all things). The former physical sound waves happen regardless.

It's like is an apple red if you don't look the apple tree. The redness of red as seen by the eye is a subjective mental state, that is like a mental shortcut that allows us to see it with the red quality. So you need an observer since that is a secondary quality (smells, colors, sounds, heat, cold. pain ect…) However, the color wavelengths that are the cause of the red quality is there regardless since it isn't dependent on an observer to translate the waves into qualities such as color.

Primary qualities that are direct are edges, lines, hardness, softness, smooth, ect… These direct qualities don't need an observer because the mind perceives them as they are, and doesn't need to "translate them" into mental short cuts like secondary qualities are.

But however, if we are all in the mind of God then everything is accounted for even the sound of a tree unseen by anyone but God.