Excellent question! It actually depends on the glass. Crystal works because the way the atoms line up in crystal makes the wine glass vibrate in just the right way. In glass wine glasses, the atoms line up differently, and they don't vibrate the right way. That's why crystal wine glasses "hum" when you run your wet finger along the rim, and glass wine glasses don't.

Singing really loudly (or making any noise that's loud enough) causes the crystal glass to break only when the song (or other noise) is loud enough to cause the crystal glass to vibrate so hard that it can no longer hold itself together. The pitch and loudness of the sound necessary to do this will vary depending on the size and thickness of the crystal glass. Larger or thicker glasses are stronger, so it will take a louder volume and probably a higher pitch (higher pitched sounds carry more energy) to break the glass. Smaller and thinner glasses will break at lower volume levels, and probably at lower pitches. The design of the glass probably affects its structural strength,too.

Either way, the sound has to be very, very loud. Don't try this at home, unless your parents like really loud noises and don't mind losing their crystal wine glasses!