I had almost the same problem but my text sound would work then not work radomly it seemed. Did a restore (my backup file was lost/corrupted so had to re-istall everything..very annoyed with that) and nothing changed. One of my friends developes apps for the iPhone. He told me that theres a lot of apps that change the sound settings and even after you hit the home button they still run in the background. There is a force quit by just holding down the home button until the app closes. This way it forces your apps to close completely (including running in the background). I close all my games/apps/iPod/Safari this way and havent had any problems since. Not to mention this will stop the iPhone from freezing up or crashing due to low memory from background running apps hogging memory.
Hope the 2.1 update addresses the real issues behind all these bugs and doesnt create more issues.
Add another one to the count mine does the same thing... Turning it on and off does fix it for a short time but it comes back. Only thing I found to help is to alternate between different tones I use tri-tone still that stops working then switch to glass till that stops and then chime.... I'm hoping the 2.1 update fixes this.