Same problem here on a white 920 after only one day's use.
Was all ok until I played a you tube video and the sound cut out after about 3 seconds.
After that, no sound on the phone at all. No key press clicks, tones, ringtones - nothing!!!
I did a factory reset, but still to no avail.
Discovered that lightly tapping the bottom of the phone brought it back,but keeps intermittently breaking up.
I am on my second 920 now.
The first had an issue with data over 3G, battery problems and terrible sound quality over Bluetooth.
All of these were cured with the second phone, but now this new problem.
I am really beginning to doubt the reliability and quality of these units.