I figure it could be one of three things. Either he is talking to someone on his cell phone, talking to a person that is not even there, or you are hearing voices that are not real.
I hope it is a cell phone that he is talking on because if you or him are hearing voices that do not exist then that may indicate a mental illness.
Yep, I agree with Mammo. Cell phone is being used. Or he's learnt to chant.
Sounds fishy to me! I agree with ILADVOCATE. If this were happening at other times.... But only at night when you are downstairs? Is there a way you can tell him you're going downstairs, but don't and stay closer and listen? Start probing, if he's using his cell phone, check the bill and see if there are numbers called frequently that you don't recognize especially at that time of night. If you have "redial" on your home phone and he is using this phone, hit redial and see who answers. This isn't in your head, he may want you to think it is, but it's not. Get snooping.
If you experienced this at other times I would say its worth speaking to your provider about but if it only happens at this time it might be real and since he might know (if you are) that you have a diagnosis he might be manipulating you. Firstly think if you hear things that aren't there at other times. But if not look at the list of numbers called at those times and see if he's speaking to someone he might not want you to know about.