Why can't I disable my iPhone's (iOS 5) new mail sound? I have an iPhone 4 running iOS 5.

In the last month or so, it has developed an annoying habit. When a new mail arrives it makes a "boing" sound and a vibrate.

It does this, even though the New Mail sound setting in Settings is switched to None. It does this, even when all sounds are switched off, via the mute button!

It is very annoying! Does anyone else see this bug, and does anyone have a suggestion (short of a clean install) to fix it?
I had this problem. It was caused by the Gmail app. If you have it installed, go to your phones Settings and find the Gmail app in the list. Turn off Sounds and that should fix your problem.