Starting a fresh tab and new game. I died in the first boss battle. That's fine but when it returned back to the main menu, the boss music continued as well as the main (both playing simultaneously).
I shut down the EXE and reran it. Won against the boss this time. Got a chest. Opens in the chest room (likely prior error is fixed since this happens !). Got a sabre. Going to try and play this to conclusion to see if there are any more bugs - but I think you got 'em ! :D