I love the idea of xoupling gameplay and music!
First playthrough I was super confused and thought the letters were just coming at random but then I realized that I was actually typing a sentence. I enjoyed it once I got it and especially once I realized the beat the letters were moving over the guitar to. However this is also where I think you could indicate a bit clearer when the letter needs to be pressed, either visually or audibly (or both even).
The music was nice and of really good sound quality but it feels more like background music to me. Maybe the music itself could be a bit more exciting, like the title screen music which I really loved!
Overall, a really good game which, given more development time, I think could be really enjoyable:)