In the bell jar experiment, the sound gradually grew fainter after the vacuum pump was turned on until we could only hear a feeble sound. Why?

In the bell jar experiment, the sound gradually grew fainter after the vacuum pump was turned on until we could only hear a feeble sound. Because the medium is sucked by the pump

Sound always travels in a medium 

The air is pumped out by the vacuum pump, and thus there is no medium in the jar and so the sounds get fainter.