Another solution to the original question is to search the html file for
if (soundPlayer) {
soundPlayer.pauseTune();
and just comment it out by adding two forward slashes before the "pauseTune" part, like so:
if (soundPlayer) {
//soundPlayer.pauseTune();
But be aware: every time you save your game from the Bitsy maker it will restore this piece of code. So you'll have to comment out the "pauseTune" code in the html file after editing your game in Bitsy, but before your game is played.