iPhone 4 : Notification sounds are not playing! HELP!

I have an iPhone 4, the speakers are working as I can play music. Howver when a new mail is received or send or a new message arrives although the notification display happens no sound is triggered.

In options I have it set to so both vibrate options are off and the volume slider is all the way up. Also if I select a sound for new mail for example I hear the sound BUT when a new mail is received the sound isn't played!

I'm on 5.0.1 and the sounds just stopped working last night after I put it on vibrate only as I was at a gig.