NOTE THAT THIS DEMO IS A DISCREPENT EVENT.  While it is true that sound needs a medium in which to propatate, the vacuum that is normally attainable still leaves enough air in the bell jar for sound propagation.  What is really happening in this demonstration is that by evacuating the bell jar you are creating an impedance mismatch between the inner and outer walls of the jar which hinders the sound propagation through the jar walls.  This in turn leads to the sound going away as you create the vacuum in the bell jar.  However, if you were able to stand inside the bell jar during this process, you would find that while the sound gets softer, you are still able to hear it at the maximum vacuum normally attainable.

When put into the bell jar all of the buzzers will experience a great loss of volume.  Pads of sorbothane rubber are used to eliminate any vibrations.  Hook the bell to 6 volt battery. With the bell ringing evacuated the bell jar with the vacuum pump. As the air is drawn from the jar the sound will diminish. With the vacuum pump running it is hard to hear the bell. The best arrangement is to put the vacuum pump in room 58 and  run a vacuum hose from there into the lecture room.  A switch will be needed also in the lecture room so that you can turn the pump on from there.  Another alternative is to evacuate the bell jar and then shut off the pump.  As you then bleed air back into the jar the buzzer will start to be heard.