That is weird. My two guesses are that you have an app that is putting your phone into silent mode, or it's a software glitch that has copied over from froyo to gingerbread.
You could try to uninstall one app at a time and see if that fixes it, or you could do a factory reset. Factory reset solved some problems I had when I updated my phone to gingerbread. It does delete your apps and settings, but will not delete anything off your SD card so your pics and music will be safe.
