I am going to answer this seriously.
It depends on the level of their hearing impairment.
If they are totally deaf, they can not hear their own voice so much as they can feel it. They can feel the vibration in their chest and their head, thus allowing them to feel the difference in vibration frequency when their voice obtains different pitches and tones. So deaf people do have a vague idea of what their voice is like, and roughly how they are using it.