Sounds like Apple has varying hardware AND software issues with iPhone 4's audio input.
In my case and in many others', turning off the phone and even hard resets, which turn off all background apps, too, didn't/doesn't help. I almost never use any other voice apps, plus physically smacking the back of my iPhone restored audio input, greatly implying a hardware problem.
With my iPhone 4, the only way I could see that the problem still might be only software-related is if the software driver(s) that controls one or more audio-input-related hardware misinterprets/ignores/loses contact with hardware until a physical jolt causes the hardware to send a signal/data large enough that the software driver notices. This seems unlikely to me, so I currently greatly lean toward hardware problem as the culprit.
Isn't this pathetic that Apple leaves its consumers to figure this !@#$ out!
Here we are on Apple's own forum, stumbling through the night of audio input problems on a PHONE as its manufacturer sits idly by eating a sandwich while reading our posts. Apple wants to be a consumer electronics company and not just a computer company, but that's a pipe dream FAIL.
If Steve Jobs goes, Apple will experience the 1990s all over again. But at least next time Linux/UNIX will have better support via Droid, which has the advantage of not being propped by an inherently unstable cult of personality benevolent dictatorship.
Here's the definition of telephone from dictionary:
"...a system that converts acoustic vibrations to electrical signals in order to transmit sound, typically voices, over a distance using wire or radio." But to Apple it's no big deal that its PHONE often can't do that!
