Do people in coma hear you?

No. In true coma there is no recognition of anything in the environment. Sometimes people who have been in a coma remember conversations, but many times these never occurred. Dreams and other neurologic functions can cause false memories. However, it's a good idea to always act as if the person can hear what you say. When coming out of a coma some conversations may be remembered.
Hard to know. It is hard to know if people in a coma can hear you. Health care providers often encourage family members to interact with their loved one in appropriate ways when they are comatose. Part of the difficulty in answering this question is that depth and cause of coma may vary, and memory is impaired during coma so people might hear but not be able to recall what they hear.
Difficult question. Coma is a difficult behavioral state to define. Most people think it is a sleep like state with lack of immediate reactions to stimuli. There are states like that in which functional MRI studies have shown some brain activation to auditory stimuli, even though there seems to be no behavioral response. The question cannot be settled in most cases.