As for many instruments resonance, or the vibrating due to an equal frequency, causes the sound. For an opera singer to break a glass with just his/her voice they would have to match the frequency or pitch that they sing at to that of the glass. When the opera singer hears the frequency of the glass, being trained they can match it to the sound that comes out of their mouth. This causes the glass to vibrate and eventually shatter once the pitch is perfectly met. For a guitar, the strings are what causes the music. When you put your finger on a string and shorten the wave length, the frequency then increases and you get a higher pitch out of that cord. This is true with most instruments such as trumpets, trombones and pianos because as you slide or press, you shorten or lengthen the wave length and in turn are changing the frequency...without physics, music wouldn't be possible!