Really enjoying this! A couple things: would it be possible to add importing the json it exports? Also, I noticed I can layer sounds by dragging them into the same grid spot, but then only the first one is visible. Would it be possible to have grid spots shared by multiple sounds split the image so you can see all of them? So if a spot has a red and a blue sound, the square would be half red, half blue, and go up to four colors/sounds total (higher would probably become too hard to see)? Or at least an indicator that there are multiple sounds in that spot. And maybe a pop-up when you hovered over them so you could pick which one you wanted to work with, in case you wanted to delete or move one, without having to move/delete the others.
The overlap probably isn't common for pitched instruments, but when you add drums it can happen. :)