I don't know for sure what your Mum's symptoms mean, but like heartbroken, just letting you know that someone is reading (this is a bad time of the day/night to get responses as the majority of members are in bed asleep).
However I can also tell you that it is not overly rare for dementia sufferers to 'see/hear/interact with' people/things that non-dementia sufferers can't so maybe that news will at least make you feel less alone/concerned.
My Dad who has dementia, used to talk about people in the room that we couldn't see, sometimes they were friendly people, sometimes they were people who were no longer alive, sometimes they were people/things that he thought were 'bad' or even seemed to scare him.
The thing we found, and I think others on hear have also often found that it is best not to argue with the dementia sufferer about this alternative reality that they have. They will get defensive if you tell them nothing is there, they will get hurt or they will get angry. I used to be able to compromise with my Dad to a certain extent however and tell him that even though I actually couldn't see them, I had no doubts that they were there for him - I used to use this tactic only when he was scared or angry at whatever he was seeing and I am not sure that it can work for other people, it may be that Dad was still in some ways able to rationalise. It helped him in these situations because I could show him that the person/thing wasn't scaring me or offending me, and that seemed to calm him.
I am not sure from your post however whether your mother has been diagnosed with dementia at this point, or whether you are suspecting it because of her symptoms? Maybe if you described more about what her short term memory loss was like (i.e. is she just forgetful or does she lose chunks of whole days?) and described more about her hallucinations (i.e. is she only hearing the singing or is she seeing the singers as well? Does she talk to the halluncinations?) someone might be able to help more.
Best wishes in finding the answers that you need.