Welcome, catterkane.
It's so hard to answer your question. I can only share my experience. I've never heard voices due to alcoholism. Mental health issues can be complex. Your Mom may be trying to get rid of the voices by drinking. Hard for me to say. If your Mom is on medication and drinking this may exacerbate her symptoms. Any concerns should be shared with her Doctor.
I agree with Anna. Look up Al Anon in your area and attend some meetings. This would be a great way to get help coping with this situation.