Samsung Galaxy/Android Notification sound not working?

I have a Samsung Galaxy Ace Style, running on Android.

I've got a Notifications folder set up, in which is an MP3 file. This plays just fine when I access it through the phone, and can be set as my Notification tone, however when I actually get a message through, it doesn't play. The phone will vibrate (I checked the box to vibrate when ringing) but not play the Notification sound.

Sounds work fine on everything else including phone calls, its just text messages that don't work.

I've tried looking online and found no help.

The only two suggestions I found were to remove AT&T (which my phone doesn't have), or swipe along the windscreen until I find a circle with an i because this is the notifications turn on and off (again, can't find on my phone).

All help appreciated.