Yep, I noticed the sound issue on my unit when I got it. With the cover on i get distortion cause of vibration, especially when the moving the phone, say swinging my hand with it. With the cover off, no distortion. I hear much better levels and what not, not so much more volume but a clearer sound when the volume is high.

My way to kill it was the stop the vibration of the cover was to tape a lil pad under to the cover, like the one already there that put extra tension when the cover snaps into place so it wouldn't vibrate. It helped.