I always wondered - could she really do that?
Scientific American addressed the issue:
Only the finest leaded crystal is dainty and resonant enough to break at volumes that some people can produce without amplification—upward of 100 decibels. A famous commercial from the 1970s showed Ella Fitzgerald shattering a wine glass with ease through Memorex speakers, and the trick has been repeated many times with amplification. The principle of directing sound at a brittle object is used, for example, to break up kidney stones—except doctors don't bother to find the resonant frequency, preferring just to blast the stone with lots of sound energy (and if a singer were as loud as, say, an explosion, she wouldn't have to find the resonant frequency to break a glass, either). Yet, it seems that until a couple of years ago there was no proof that any person had ever broken glass with his or her voice alone.
Then in 2005 the Discovery Channel television show MythBusters tackled the question, recruiting rock singer and vocal coach Jamie Vendera to hit some crystal ware with his best shot. He tried 12 wine glasses before stumbling on the lucky one that splintered at the blast of his mighty pipes. For the first time, proof that an unassisted voice can indeed shatter glass was captured on video.
Vendera's glass-breaking wail registered at 105 decibels—almost as loud as a jackhammer. Not many people can muster the lung power for that kind of noise. Opera singers train for years to build up the strength to produce sustained notes at volumes above 100 decibels.
Here is the video of Vendera's accomplishment.