Issue resolved.
I found a slightly alternate way of preventing the screen dragging, which does not seem to interfere with the sound. My event handlers:
canvas_html.addEventListener ("mousedown", function() {mouse_down (false)}, false);
canvas_html.addEventListener ("mousemove", function() {mouse_move (false)}, false);
canvas_html.addEventListener ("touchstart", function() {mouse_down (true)}, false);
canvas_html.addEventListener ("touchmove", function() {mouse_move (true)}, true);
canvas_html.addEventListener ("touchend", function() {mouse_up (true)}, false);
document.body.addEventListener ("mouseup", function() {mouse_up (false)}, false);
document.body.addEventListener ("touchcancel", function() {mouse_up (true)}, false);
document.body.addEventListener ('touchmove', function (event) {event.preventDefault()}, false);
I commented out the "preventDefault" in my mouse_move() function and added a new event handler for 'touchmove' at the end of the list, just to handle the "preventDefault".
This was just a wild guess, and I'm not entirely sure why it works this way and not the other way. It does, indeed, work, however, and I now have both sound and a stable screen.