From another forum---
People have long wondered: "If a tree falls in the forest and no one is there to hear it, does it make a sound?"
I have found an explanation:
Interestingly enough there was recently undertaken a scientific experiment to establish that there was a sound even though nobody heard it.
A bunch of plants were arranged behind a totally sound proof glass. There was a recording of "Mary had a little lamb" played and an extremely high resolution video camera recorded the movement of the plants as the song was played. Then through a program that transformed the visual movement of the plants back into sound, the audio results of the experiment were played. While pretty low quality, the audio was clearly discernible as the song "Mary Had a Little Lamb."

So even though there is no one to hear the tree, the surrounding forest reacts to the sound.
Now you know.

Sound proof glass? Plants' hearing is faaar better than mine! I play music in my greenhouse a lot.

I studied this, googled, and found out the person who wrote this got the basics wrong. They took high resolution videos, and coordinated vibrations of the leaves with the words of the song. I don't remember finding an explanation about that bit about sound proof glass, and I'm not spending any more time on this. Perhaps a way for men to joke about their wives saying they're always wrong!