This is a weird suggestion but do you have anything near your bedroom that makes noise? At my old house the AC unit was right outside my bedroom and for a couple of years I would hear voices periodically but not in any pattern.

Then one night I was in the basement when the unit came on and the different perspective made me realize that the voices I"d come to take for granted were simply the AC unit running.

Is there anything like that which could translate into voices in your tired, bipolar brain?