Help with notification ring-tones:

[kwitel]

I seem to occasionally and for no apparent reason, be losing the ring tone associated with my text messages.
I have the box for ringtone checked off, everything looks good but all I am getting is vibrate.

So, If I restart the phone...I get the ringtone back. But then after a couple of hours, it magically goes back to just vibration and I have to restart the phone again.

Any ideas?