They snap shut because they have sensors that will make the trap snap shut when they are disturbed. The reason why they snap shut is because of turgor pressure which is pressure that is built up in the cell walls of the plant from water.
When a fly touches one of the tiny hairs, the trap automatically gets triggered to shut.
If my edit to the question was correct, you cannot hear plants because they do not produce noise. If you prefer, we can hear plants if you consider wind going through it, or the snapping shut of the venus flytrap.
An insectivorous plant (Dionaea muscipula) of the coastal plain of the Carolinas, having sensitive, hinged, marginally bristled, two-lobed leaf blades that close and entrap insects.It has traps and has tiny hairs (censors) on the inside of the traps. Flies and bugs are attracted to the scent of them, so when they land on a censor the trap shuts, eats, and then it will digest the fly or bug using special enzymes.The Venus Fly Trap - is a carnivorous plant. Its leaves form two halves of a hinged 'trap'. When an insect crawls over the surface, it trips 'trigger hairs' on the leaf, which snaps shut in a split-second. The insect is then dissolved by digestive enzymes.See the related link for a photograph in Wikipedia.
It is haré to answer so shut up
It is not a mouth. It is a leaf. On the leaves are trigger hairs that trigger the leaves to close. The leaves lie open in wait of prey. Then their colors attract an insect. The insect brushes against one of the hairs. It brushes against another hair and the trap closes. It can also brush against the same hair two times.
The triggers tell the plant to shut its trap.
When a bug lands inside of a venus flytrap, the plant senses it and snaps shut. Then the bug starts getting eaten.
The Venus Flytrap is unusual to normal plants because it is a carnivore (it digests meat), it uses the extracted nutrients (mainly nitrogen) to supplement it's nutrient intake. It also has sensory hairs in its trap so that when it senses movement, it will trigger the trap to shut.
The Venus Flytrap hunts by luring insects into the jaws of its trap. Inside the trap are glands that secrete nectar. This tricks the insect into thinking it has found a flower. The trap has small sensor hairs that are triggered by the moving insect. The sensor hairs triggering causes the trap to shut and trap the insect.
Those are called 'triggering hairs' that do exactly what it says on the tin! They 'trigger' any movement that enters the leaf, and cause it to shut like a trap, hence the name, Venus "Flytrap".
I do not think that it has a specific name for only that. I am not a plant professional but I can tell you that number one; they aren't leaves and number two; It should only be called "the head of the Venus Flytrap closing". It also does not need a capital V since it is not the name of a planet, but the name of a plant (Venus).
inside the Venus fly trap plant there are hair pointing downwards when the insect enters the lid closes to trap the insect and it gets entangled in the hair . the cell of the plant secrete digestive juices to digest the insect.
Venus flytraps have small sensors or hair that line the inside of the plant. When the fly lands on the plant it snaps shut and the fly can not escape. Try this out: Stick your finger in the flytrap don't worry you will be able to escape though.
When a bug or fly comes into one of the Venus Flytrap's traps, tiny hairs will sense movement and will enclose the trap on the animal to be digested. This process normally takes around 10 days before the trap reopens.
When a fly touches one of the tiny hairs, the trap automatically gets triggered to shut.
1/30 of a second
If two trigger hairs are touched by an insect inside one of its traps, the trap will snap shut.