Actually it might be the other way around.
low self esteem make her talk in a deeper voice.
Of course not consciously.
But there have been seen some indication that low self esteem make girls voice deeper, and guys voice higher.
Of course it is also a technical problem.
I know a girl that had the problem, and the school send her to a talking specialist, after a couple of months her voice was like normal. Perhaps still a bit deep but in the normal range.