I use an iPhone 4 to record videos with the Camera App. Suddenly, the 5 most recent videos got muted sound for no reason at all. The rest of the Camera Roll still has audio. Just those 5 videos got their sound deleted.
I record a new video, and it also has no sound. Why did that happen? I know this isn't caused by dust in the tiny microphone hole by the headphone jack. That would only make future recordings have no sound.
I record a video and shout into the microphone. I play-back the video at maximum volume, and zero audio comes out. That's how I know it's not a problem with the microphone.