I have an iPhone 4 (Verizon, Model MD146LL ) and I just upgraded from iOS 5.0.1 to iOS 5.1 (9B176) using Settings > General > Software Update.

Immediately after completing the update, I noticed that my phone did not make the unlock sound as I unlocked my phone. "Oh, the volume must be down", I thought. So, I made sure my mute switch was off and pressed the + button to increase the volume. Unfortunately, I was greeted by the empty ringer volume popup:

"Apple must have changed the default setting for the volume buttons ^^ "... Surely, I could change it myself! So, I head over to Settings > Sounds:

Oh, the settings are right. Hmm... What happens if I move the volume slider?

Rejoice! MARIMBA!

Ok, so my phone can play audio. Can it play music or watch a video?

No!

No!

NO...

Other controls:

Hoping that this was just a temporary glitch, I tried the following:

Switching Change with buttons on and off

on and off Turning my phone on and off

Hard reset (holding Home + Lock until it reboots)

+ until it reboots) Plugging and unplugging my earphones

Plugging and unplugging the USB charging cable

Plugging and unplugging the USB cable seems to fix the issue. However, as soon as I lock the phone, the sound and volume controls are disabled again! Due to this behavior, I'm convinced that this is a software bug and not hardware related (e.g. faulty headphone jack).

Has anyone else experienced this? I'm afraid I'm going to have to do a clean restore or downgrade back to iOS 5.0.

Update 1:

Settings > General > Reset > Erase All Content and Settings was unsuccessful, even when setting up the phone as a new device (no backup). The bug is still present. I will be trying a clean restore from iTunes soon.

Update 2:

Restoring as a new device using iTunes was also unsuccessful. :( From my research, it looks like this bug not specific to my model:

Update 3:

After further analysis and comparison, I think that this bug is related to the External Accessory Framework (or similar components). When you are plugged into a compatible device, the controls are hidden, similar to my screenshots above. This may also explain why plugging and unplugging the charger temporarily fixes the problem. "I'm unplugged; I should enable my volume controls now."