I spent a year of training working with patients in comas, and those who had come out of comas but with severe brain damage – considered “vegetative”. Of the ones who came out of long term comas ( 3 weeks to 4 months) ALL of them who recovered most of their mental faculties reported that they heard conversations in their presence, were aware of people coming and going in their rooms, and could even repeat some conversations (much to the embarrassment of some of their caretakers!).
Those with severe brain damage (vegetative) usually responded to verbal and tactile stimuli, and often responded more vigorously to those they recognized who were with them on a daily basis, and especially to family members, smiling, moving their hands or arms, making eye contact, etc…
I would treat anyone in a coma (especially the terminally ill) as being able to hear and understand what is said in their room. When I worked Hospice/Terminal Care, I learned that hearing is the last faculty to fade away at death. We spoke to those in comas all the time, always explained what we were doing when getting ready to turn or bathe them, just as we would with a patient who was conscious. I believe they can still hear, and often understand a lot more than they seem to, even with severe brain damage!