The most obvious way to give sight to blind and hearing to deaf is to give them a replacement organ for these. In order to do that we would need to understand how our eyes and ears encode sensory information into electrical signals which then go into the brain. My question is, how close are to understand this mechanism at this time?
I think the encoding is quite well understood, and in fact there are artifical cochleas available for deaf people: https://en.wikipedia.org/wiki/Cochlear_implant
For retina replacements, I think the hurdle is establishing the connections from any device to the nerve. There are roughly 120 million rods and 6 million cones in the human eye. (Per Wikipedia: https://en.wikipedia.org/wiki/Photoreceptor_cell ) These connect to about 1.2 million optic nerves, so making the proper connections would seem to be a non-trivial matter.
In general I would say a lot is known:
Seeing starts with the photo-receptors. See "signal transduction" section of the Wiki: Signal transduction works though rhodopsin and subsequent closing and opening of ion channels.
Then the signal is transferred from the optic nerve to the brain.