When the dome is filled with air, the sound of the bell can be heard quite clearly. When there is less air in the dome, the sound becomes muffled.

Sound is a mechanical wave motion that requires a medium in order for it to travel. The sound wave makes the air molecules vibrate and the vibration is transferred by the molecules to your ears. In a vacuum, sound cannot travel because there are hardly any particles to vibrate.

The siren on a fire engine sounds shriller as it is approaching than as it is moving away from you. This is the result of the Doppler effect. The Doppler effect is the change occurring in the frequency, phase or wavelength of wave motion, caused by a change in the relative distance between the observer and the source of the sound waves.