SMS Alerts Not Ringing on 3GS After iOS5 Upgrade?

Hi folks,

Odd issue with SMS ringtones after upgrading to iOS5. Despite the phone not being in silent mode and a text message tone being set up, my phone does not play the new text tone when I receive a new iMessage (seems to work for standard SMS messages though). This is for when the phone is locked, by the way; I know that there are no sound alerts configurable for the main notification area.

This is annoying as it seems to only pop up an alert on the lock screen without actually giving me any audio notification. So if my phone's in my pocket or even if I'm just not looking at it, I don't know that I've gotten a new message.

Any idea what I might have configured wrong?