Maybe it's something I'm doing but my Droid phone app goes to silent mode by itself. Anybody know about that? I use AppKiller but I see nothing on it that controls phone sounds.
Second, go into the Setting Menu on the home screen. (Or just click the Settings icon) Scroll to Locale and check your settings. The phone does have the option to set certain volumes and settings at certain times for notifications. Might be simple as that. At least it's a starting point.