You have to change your sound profiles. Look for the icon that looks like a speaker and click on it, if it vibrates twice before it rings, then you profile is set to loud, scroll down to normal and that should fix that. you can customize all of your ring tones and alerts by scrolling all the way down to advance in that menu and modify how many times it vibrates, the tones and ringers for sms, mms, all the different messengers, make active profiles for different people, etc....you just have to experiment a little bit. Good luck.
