The sound is lagging behind my typing by at least a second. As a result, I hear nothing and then bursts of several instances of the sound playing at once, which sounds awful. As others have pointed out, the lag might be because you're invoking local storage after every keyup event. I see the merit in that idea, but as AskHugo suggested, it runs a lot smoother if you save every 2 seconds. I don't think anyone would complain about losing a maximum of the past 2 seconds of their work.

Sound effects are always going to be a polarizing feature (some like it, some hate it), and the comments on this post prove that. You should provide an easy way to turn sound effects off.

That aside, great job with this. I am a big fan of beauty through simplicity.