Can you post a .capx which can't stop audio? Last I tested it it was working OK.
Firefox can't loop audio, it's a browser limitation.
Browsers have really terrible audio support and to try and make it work as well as possible the Audio object currently uses three different audio APIs. It's a bit of a nightmare, and patchy browser support means it still doesn't entirely work. It's something HTML5 needs to improve on, it's not something we can fix our end.