Corsica,
I would suggest having your mother try sleeping in another room and see if there is any
change with the voice.
You can also use ear plugs; they will or should help lessen the intensity or even block it
out depending on how loud this voice is.
You can try leaving a television or radio on throughout the night and day time on a low to high volume.
Have you considered in the mean time to use sleep aids or mild tranquilizers for your mother to be able to sleep?
You can tell the doctor she's suffering from insomnia just for example and do not have to go into details.
I would also suggest searching the internet for a local paranormal team who should be
able to put you in touch with a demonologist/ Priest or qualified person capable
of performing exorcism prayers and blessings if needed.
The paranormal team may want to try and investigate to see if their is activity in the home
where your mother lives also. Don't let this stop you from proceeding; it's not a big deal, but
you can always tell them you would prefer someone they can refer for exoricism prayers or house
blessings/cleansings or deliverances also depending on what your mothers faith is.
Before allowing anyone in, question them thoroughly and you should get a good sense about
them; most are reputable and out to help and again they do not charge for their services.
They should not charge you any money for their services; remember this.
When people speak about demons or being possessed they usually are not possessed, but rather
it (the entity/energy) is attached to the person being bothered by it.
It may not be a demon, but it could be a very persistant and mean energy.
You and your mother can also demand that it leave in the name of the Father/or Jesus or God.
You can also use the words; I cast you out in the name of Jesus, the Father or God whichever suits your needs.
Refer to the Bible also and the internet for prayers and comfort.
User Super Z has exorcism prayers listed under Research and Home Remedy that you can play also.
http://forum.intervoiceonline.org/showthread.php?t=767&page=5
http://forum.intervoiceonline.org/showthread.php?t=767
You can contact me thru email and tell me where you live also and I could possibly find
someone in your area who may help you if you do not find anyone thru the internet.
Good Luck and May this thing be gone from your lives forever...