A mad/ delusional person may hear and see things within his/ her 5 senses that may be very different to what you or I perceive with the 5 senses, so that is the mad person’s Truth as far as what his/ her sense tell him/ her.
We can only know Truth that is immediately within our 5 senses in our immediate environment, not something far away as a tree making a sound.
Does my perception of the colour “red” equate to your view of “red” or could my view of “red” be your “blue” from your perspective? We don’t know what happens after death because the brain is destroyed and it is the essential component to sense Truth in our immediate surroundings because it computes signals from the 5 senses. For all we know, we could be living in this dream-world (which me and my perception of you are living in) and then when we die, we don’t wake up from that dream-world because there is no functional brain to process the 5 senses we have for our immediate surroundings.
To sum it up, we only “exist” because we think we exist, all because of our brain processing the 5 senses and interpreting our existence as Truth based on what our 5 senses are telling us.
Being aware of our own Truth through our 5 senses makes us conscious and in doing so builds up our Reality or what we think of Reality.
Therefore, the tree, the forest and the sound made when the tree falls does not exist because there was no sentient beings like ourselves to perceive it to exist or to be “true”.