there are two proffessionals you should consult, with your daughter there to answer questions they have, and with her knowledge and consent.
a) a physcologist
b) a priest
My reasoning for this is two-fold. Mental illnesses can develop at any time, making that a real possibility - and the sooner diagnosed, the better treatment options and the more likelihood of living a perfectly normal life despite that particular triel.
But I am not mentally ill, and I have heard a voice before as well. When I was thirteen, I prayed for the first time in my life, and I was answered - by a female figure, a mother figure, who stepped in and taught me to trust God and has since stepped out. Hearing her voice was different than I would’ve expected, because it was more like ‘feeling’ her voice than ‘hearing’ it, if that makes any sense at all. It is an experience that is very difficult to convey. But it certainly was not the sign of a mental illness, rather it was a new beginning in my experience of being called to be close with God.
Your daughter is very blessed, either with something to make her grow spiritually a lot more directly, or with a trial that, handelled well, could lead her to help a lot of people who need help the most in our society.
Most of all, whatever happens, stand behind her, love her, support her, and and let her know that your love of her and respect for her will not change whatever happens.