The brain fills in what it expects in your peripheral vision. For example, you can see colors in your peripheral vision, but it is only an illusion made in the brain.
This Ted gives some good examples https://www.ted.com/talks/anil_seth_how_your_brain_hallucina...
I guess my vision may indeed be abnormal as somebody else above suggested, since I don't get the blid spot in the central vision in the dark, either.
As for the sound, I think you just learn to hear the sine waves as speech - I tried to convert a part of audiobook (that I haven't listened to before) and I can understand it a little bit. I used praat with this script (it doesn't seem to like long sounds): http://www.mrc-cbu.cam.ac.uk/people/matt.davis/sine-wave-spe...