I'm in a conference so I can't test this... but can someone with a 4S give this a try: Activate Siri from the lock screen by holding the home button. Say any command or sentence. When Siri is done talking, press the home button again and unlock your phone. Is the volume of the unlock sound (and all other sounds) still ridiculously loud immediately after using Siri? This bug has been annoying me for almost a month.
I still have this problem after updating. Bummer...