All, I've been working with Apple developer support on a suite of potential issues related to sounds (audio cues) that seem to be flaky in iOS4. These issues are noticed by me on my IP4 but may also be occurring on other devices running iOS4. Here are some threads that have reported some of the issues I'm speaking of: Mail sounds broken Volume issues Weird lock sounds And there are several others. The defect I have open with Apple is specific to another issue which may be related: Audio cues (lock sounds, keyboard clicks) just seem to go away and stop working from time to time regardless of the use (or not) of the vibrate switch. Once they go away, somethings it takes multiple reboots to get them back. I have two working hypotheses I'd like help validating: 1) The problem comes from 3rd party apps that are using the new background audio capabilities in iOS4. Specifically, once an app such as Pandora or Wunderradio has been paused or placed in background mode, you may lose these audio cues. I'd like others to experiment with this specific scenario - Verify your lock sound is working, run some background audio app (Wunderradio seems particularly prone to causing this), pause it, exit via home button. Re-select the app from the app switch dock, exit again, etc. At any time do you lose the lock sound or keyboard clicks? 2) The OS is getting confused on the state of the vibrate switch. There are a number of scenarios here based on position of the physical switch and options selected in Settings. It seems to me that I've had cases where no background audio was involved (scenario 1 above) and still lost audio cues. I think this happens when you're changing the position of the physical switch often. Example. Switch in the up position (sound on) and everything is fine. Switch is moved down to vibrate mode and sounds go away as they should. Then put switch back up and you get no sound. Not just audio cues but sound from 3rd party apps too. Anyone else seeing this behavior? Thanks.