My mother is 92 and last year started to hear a man's voice telling her things about her family tree. Any advice?

I would talk to her doctor about it (or consult with a geriatric doc) -- there may be meds to help with the auditory hallucinations.

If my mother had a UTI, her hallucinations became more intense. Has she been checked?

If the hallucinations upset your mom then it is more distressing to you and her.
However if they don’t distress her, I would not try to convince her of anything different.
Auditory hallucinations are not uncommon. Some people hear children or someone singing or a music box. Some people claim they have kept them from making mistakes like walking in front of a car. I have a family member who thinks she knows when someone is in danger and she will act on them, call the police etc. Even when she is “on her meds” she seems to hear them but is less likely to speak of it. In other words is less likely to act on them, is calmer. When I first heard of my LO’s hallucinations, I read a great deal about them. This was years ago. There are even groups that meet to share their experiences.

Does your mom have health issues, take medications? Did something traumatic happen before this started? I would be very hesitate to give her meds to stop them if they don’t upset her but it is always good to have a checkup, UTI test, etc as there are illnesses that can cause hallucinations.