On the second level, the instrument all the way to the right (bass I guess?) is not visible on a 5:4 monitor, and the mouse can't access it - so the level cannot be passed. I could only see the instrument on game screenshots here. Generally, the game is a little cut off at the sides on a 5:4 monitor. This is on Linux by the way, in case you can't reproduce the bug on another platform.
Hi thanks for letting me know. I'm sorry you ran into that problem.
I've had reports about other screen ratios where some levels become impassible (e.g. 3:2 monitors). So I'm planning to do some thorough testing of various screen ratios and trying to fix all those problems. Hopefully I'll be able to do that before the next update.