When a tree falls, the crash causes ripples in the air. These ripples can be detected by a nearby creature having ears and a brain. It takes ears and a brain to make a sound out of ripples in the air. The ripples occur in the air; a sound occurs in the brain.
So, in your "millions of years ago" scenario, it is true to say the tree caused ripples in the air. If there were a hearing creature nearby, it is also true to say that the tree made a sound.
But let's address your primary question, which is, did the event occur at all, given that there is no record of it today.
In the world as we perceive it, it turns out that the situation at this moment determines the situation at the next moment. If I drop my coffee mug, it will fall, and the next moment there will be a mess. This seems to be a basic law of the universe, the law of cause and effect.
From this we can also say that the world as we perceive it was determined by the world as it was at the previous moment. That world was determined by the moment previous to it, and so on into the far past. There is a very definite chain of events.
Now we can begin to see that every event in the past is meticulously recorded in the moment that comes immediately after it. Everything at a given moment is not only a consequence of, but also a record of, what just happened. That moment in turn is meticulously recorded in the next moment, and so on into the far future. It does not matter that we are unable to comprehend the present clearly enough to be able to read the record of the past in it. The record is there.
We are able to comprehend some of this record. Your tree rotted away, but another fell in a bog and was preserved as a fossil imprint. But even your tree which rotted after it fell, so that we do not have the ability to reconstruct its falling as an event, is still linked in an unbroken chain to events afterward, such as decomposition, reuse in other creatures, burial in the silts of time, and metamorphosis into rock strata. It still happened. This moment contains within it the meticulous record of the tree falling.
Nothing is ever forgotten.