They hear every single bite.
"Food" for thought: I see this as an obvious reason to discuss the anthropocentric privileging that goes into refraining from eating one form of life and not another. As it stands, vegetarianism is most often founded on the refusal to eat animals that remind us the most of ourselves. Ultimately, no matter what we eat, we are consuming life, living off an-"other" thing (or being’s) energy. To exist is to take the energy from another — to take life. This is very, very problematic and, as hyperbolic as it sounds, we might consider that the only way to truly refrain from causing harm, or pain, is to cease "being" (and consuming) altogether: an inconvenient perspective that would completely reframe the discourse on suicide, traditionally seen as immoral, as perhaps the only moral choice.
More food for thought: Why does everyone assume that most vegetarians are vegetarian for ethical reasons? Has such a study ever been done? I am a vegetarian for the environmental benefit and for the most part when I meet new vegetarians they are for those reasons as well
Also, there’s a certain degree to the ethical question. The vegetarians I know who are vegetarian for ethical reasons say it is because of animal cruelty, i.e. not eating beef because most cattle in America are raised in space only slightly larger than they are and fed grain which they cannot properly digest, etc.. Saying that any pain at all to a plant makes it ethically equivalent to the confinement and abuse of animals paints the world in black and white strokes. Honestly I’d say that argument is a strawman, misrepresenting the opinions of most vegetarians.