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.
Geordi also has an effect he calls blurry after images in his visor. It's what initially made him dizzy. It's made out that they are both some kind of echos from previous loops