First, if I'm up in time, the only way to stop the alarm is to shut down the computer (since the task runs whether I'm logged in or not, I have to shut it all the way down). Second, the file only plays one time, it doesn't continue.
One way to fix this could be to make your script into a service. This way if you got up early you could just go in to the task manager and stop the service. I'm not entirely sure how to manually configure a service in Windows but if you don't mind installing additional software this should be trivial to do using NSSM (the Non-Sucking Service Manager)