Loved this ep.
I just do not understand how the doctor was able to hear voices. It would have made more sense if it happened to Troi.
The voices were a physical manifestation -they were actually there, not in Crusher's head. Geordi says sensors had picked something up and other crewmen reported a problem too. It was initially assumed to be a problem in the Com System. Crusher's room just happened to be in the right place and time for her to hear -and later record- the voices.