From what I know the nightmare-ish visions and voices that happens during a sleep paralysis are due to you still being in like a semi-dream state and being scared/panicking of the sleep paralysis and since you're still somewhat dreaming manifesting those things realistically infront of your eyes is extremely easy since you're scared.

I get sleep paralysis pretty much every mornings because of failed LDing and what not but I'm so used to them that I just go "Meh" during them and nothing happens at all.