This is very interesting (and as has been pointed out in other comments, not exactly a new idea). I think its proper application, though, is in a tool for practicing reading, not for actually reading tests. One major thing this approach looses is the inherent non-linearity of text. If I miss a fact and want to back up a bit, or want to pause for a moment and think about something, I can (when reading) without even th…
Fun to make. I'm also curious about the overall benefits. I find myself distracted by other thoughts if I don't have subvocalization happening.
If you want the ability to move backwards, I feel like that's possible with a good NLP library. If you skip back two sentences, or enough sentences to match some word or syllable threshold, it may be useful. In that case, you can play a few words, pause for a second or so to let the user decide if they've found the right spot (and context switch into reading) or if they should continue going back. I tried hacking in a quick back button that went back X words, but it was very disorienting. But in any case, it isn't as interaction free as doing so when reading a block of text is. But to counter your argument: you still have to keep your position even with text or seek for your position again, so there's a downside to those benefits.
I'm not sure how Spritz handles things like this, but I felt like making my own based on what I saw on the demo really quick to get a better understanding. It's a curious thing, maybe I'll check out the application sometime.