Same thing is happening to my new iPad 2 also. I've read all the posts regarding this. I've reset to factory defaults. Now I just noticed something. I first tried a series of steps while not tethered and out of a case while the sound was not working.
1. Settings->General has 'Use Side Switch To:' set for Mute
2. Toggled the side switch off and on
3. Using the side button for volume control does not show any bars. Curious.
4. Double-tap the home button and sliding left shows the brightness slider but NO slider for volume
5. Did the hard reset to power off
6. Powered back on
7. Still no sound despite toggling side switch while set for Mute
8. Settings-> General setting to lock rotation, and toggling side switch, had no effect
9. Allowed iPad to go dark and then to sleep
10. Waking up by pressing Home button and using slider to unlock had audible click, but no sound for music
Now I did the same things but while tethered to my MBA with the iPad cable. Still out of the case. No sound when inserting the cable but display shows unit is charging.
1. Settings->General has 'Use Side Switch To:' set for Mute
2. Toggled the side switch off and on
3. Using the side button for volume control does not show any bars
4. Double-tap the home button and sliding left shows the brightness slider but NO slider for volume
5. Did the hard reset to power off
6. Powered back on
7. Now there is the charging sound when starting up. Volume control button shows bars. At maximum.
8. Double-tap the home button and sliding left NOW shows the slider for volume. Music has audo. Yeah!
9. Allowed iPad to go dark and then to sleep while still tethered and out of case
10. Woke up iPad using Home button and slider to unlock.
11. Double-tap the home button and sliding left still shows the volume control slider. Still have music!
12. Allowed cycle of sleep and waking up several times over 30-minute period, all while tethered
Now I untethered the iPad again while the sound was working. Sound still works. Side switch still set for Mute. Volume control button still shows bars. Then I allowed the iPad to go dark and then to sleep.
After letting the iPad sleep for about five minutes then I woke it up with the Home button and slide to unlock. There is an unlock sound. Double-tap the home button and sliding left now there is NO VOLUME SLIDER! With no volume slider there is no sound. Now using the volume control button shows NO bars.
My conclusion is that keeping the iPad tethered keeps the sound working, after you have reset the unit. The first sleep cycle AFTER untethering disrupts the sound again.
I have used these steps several times to duplicate the problem. Can someone else verify the results too?
UPDATE:
Just left the iPad sleeping for about three hours with the last symptoms; no sound, no volume control slider, no bars displayed when using the volume control button on the side. Came back and tethered the iPad to the MBA. Battery charging tone. Clicks for the unlock. Lo and behold the volume control slider is displayed again and the bars show when using the side buttons. At least until it went to sleep again. Upon waking up, no sound again. Grrr...
This is very very odd.