Before you read the rest of this blog go read this one. If you don't, the rest of this won't make sense. (I had read this post this morning when I got to work, and as I typed my comment, I realized it had gotten WAY too long to leave there, so I moved it here.)
As I was reading this, I thought to myself "Well, I can 'see' the words in the Bible but I don't always 'see' a tangible demonstration of it," and I wondered what the difference was between those two types of seeing. However, your point was clarified when you spoke of that very thing. Can we hear Scripture and remember it? Not very easily, most likely due to the cultivated sense of sight.
I will make one point though, that could just be tied to me (or maybe a few others): Many of the verses I know today I learned (by hearing them over and over) through a song, or even just repetition as a child (1 John 4:7&8). Those of us who grew up in the faith can probably attest to this. Yes, as we got older, we had words on screens for those songs, but I knew them without looking. And those are some of the things that come back to me easily today. How does that affect your questions about sight? Is it something that we could correct in a generation or two if we put more focus on listening? Or are we now hardwired in some way so it would take as long a time to get back as we have taken to get to where we are today? Perhaps one could argue that my point is slightly invalid because the adults who were teaching the songs/verses were reading them, and they probably know them just as well. Or maybe it's the music or the sing-song way of saying the verses that keeps them in our brains. And perhaps, these songs, although full of truth and goodness from the Bible, do not always conquer the harder concepts, although sometimes they do, you just don't realize it. (... He who loveth not, does not know God for God is love...)
Either way, I still have to say that I find it amazing that people would memorize Paul's letters (among others) because they'd probably never see them again, they couldn't copy enough for everyone, and it was important stuff. I doubt you could make all of Romans or 1 Corinthians 'sing-song-y' enough to remember it easily. Dedication, of course is the necessary step, but the motivation has to be there too, and in our culture, that's almost laughable.
As for your question about faith, I would offer one comment (as I know I haven't really even begun to touch on that): I think we still "believe without seeing" - as mentioned before, I may be seeing the Word of God, but I still don't have a tangible visual or experience for everything He says through it. I still have never actually seen God. And yet, I believe.