How many decibels do you need to break glass?
It is not so much the volume of the sound as the frequency as well. Sound at the wrong frequency can be played as loud as you like and it will not break the glass. The sound needs to be the same frequency as the resonant frequency of the glass (tap the glass, that note is it's resonant frequency). Once the resonant frequency has been found, it does not take much volume at all, even humans can do it, albeit trained singers (see Mythbusters).