Mine Throttles back when the battery is low, that part seems fine. I dont know whats up with the musical tone. The motors make noise, kinda like a tone, but If its from the dontroller than I have no ideas there. They throttle back to warn you the pack is low and to land.(so get used to landing with about 1/4 throttle)without a 2nd change at it. This is because if you discharge a li-po to much its not good for battery life and will lead to loss in duration. I'd say charge another pack if you have one or try that, or hang the pack you have on the charger and see if charges or just says its done(if you use a GP poly charge it will beep fairly soon, as it does when the battery is done). If neither of those work than call Great Planes and report the problem, maybe there tech support can help you through it if its not something wrong with the controller.