An alarm bell is kept inside a vessel as shown in the figure below. A person standing close to it can distinctly hear the sound of an alarm. 

What happens to the sound of the alarm clock?

During sound propagation, the particles of the medium vibrate and transfer the vibration energy to the particles in the surroundings, allowing sound to travel. However, no (or very few) particles can transfer and carry vibrations in a vacuum, so the sound cannot travel. Here, if the air inside the vessel is removed completely, there is only a vacuum. So, we cannot hear the sound of the alarm clock at all as sound cannot travel through the vacuum.