He's wondering if he's been dead this whole time, like Bruce Willis.
In the Sixth Sense, that is, not generally. He's pretty sure Bruce Willis hasn't been dead this whole time.
Even though he can remember being at his sister's wedding, he's not in a single photo, even though he's certain he posed in them.
The voices beg the question; what if it wasn't *his* sister's wedding? What if he accidentally crossed into a parallel reality?
And if he did, how will he get home?