Having had this problem recently, setting a timer, doesn't matter if I use Siri to set hours or manually do it myself. The first 2-3 times I set a timer for the normal vibration and ding for the end of the alert. After a few times, all physical and audible feedback for the end of the timer stops working until the clock is reset. This happened at 5.2 and continued after upgrading to 5.3. Anyone else encounter this or have any idea how to solve?