It must be difficult for you to see this strange behavior with your mom. It wasn’t clear from your letter if there are others in the home with you. If there is, ask them about this and talk to them about talking to your mom.
If there aren’t others that live with you, you may want to talk to your mom alone, when she isn’t talking to herself. Let her know that it is uncomfortable for you when you hear her doing this. Explain that it is something that you haven’t seen her do until the past year and that it concerns you. The goal of the conversation is to begin an honest dialogue about what is going on for your mom. The dialogue will give you a chance to determine if her talking to herself is odd but harmless — or if it is an indication that she has some other issues going on.