I just noticed recently that when I'm on a phone call (speaker or regular), and I get a text message to Handcent, I get the loud notification sound. It didn't used to happen before, and I don't know how to fix it?
Not only that, but you have to go in Settings > Application > Default Messaging and re-set it as default messaging app each time you get an Android or Handcent update. Even if it is already selected, you have to reset it. I always unset it, exit out of it, and then go in and set it again just to make sure. That should fix the issue for you.