[alttag]

I swear mine does it too.

I suspect it mutes alerts after a period of inactivity (but it could be something sneaky like a combination of the photosensor and accelerometer reporting dark and no movement).

[X-Istence]

It doesn't do this with alerts from apps on the iPad for example... those still go off, so I am not sure.

Although I may have become so used to the sound that I can tune it out ...