I do also sometimes pull up some classical sheet music and try to read it along with a recording. So if you get a bigger library that includes pieces I end up being interested in, your site would be useful in that respect. I would enjoy the safety of having the red bar following along to make sure I don’t lose my place.
You said on https://news.ycombinator.com/item?id=6226990 that the left and right arrow keys should move by page, and not by measure, and they’re just mislabeled. I think it would actually be more helpful for the arrow keys to move by measure. That’s what I’m used to in other programs, and I would want to do that more often. I often move back a measure or three back to replay a complicated part I just heard. Moving back a whole page is too much, and using the mouse to click the previous measure is less efficient. You could still keep the keyboard page-moving functions, but bind them to Page Up and Page Down instead.
I’ve used Transcribe! (http://www.seventhstring.com/xscribe/overview.html) before to manually mark up the measures and beats of pieces. You say you have some JavaScript tool that helps you mark up pieces, and I would love to see it, but you might also want to look at Transcribe! and see if that makes marking up pieces easier. Transcribe! uses a proprietary file format, but it’s not too hard to extract the beat times by parsing the text of the file.
Another scrolling method you could try to help people keep their place is showing two rows/systems at once. When you get to the end (right side) of one row, move the bar to the left side of the next row, and then scroll the next row up, displaying the row after that below it. I’m thinking of the effect in this video, which uses Finale Notepad: http://www.youtube.com/watch?v=CxTav4S-4Tk.