Maybe. Depends on why they are in the coma as to whether or not their hearing nerve pathways might be working. Your doctors may or may not be able to tell you. Certainly, it can't hurt to talk to a friend or loved one in a coma. Best wishes.
No. Coma is usually defined as unconscious unresponsiveness. During coma an individual cannot hear.
However during an acute or chronic vegetative state or minimally responsive state, during which patients may appear to be in coma, a patient may have memory of what is said.