A vacuum is the substance through which sound waves cannot travel, as sound requires a medium to propagate. Sound speed varies with the medium's density and rigidity, and sound travels fastest in solids, slower in liquids, and slowest in gases.
The substance that sound waves cannot travel through is a vacuum. Sound waves need a medium to travel through, such as solids, liquids, or gases because they are pressure waves.
In a vacuum, there are no particles to transmit the sound waves, thus no sound can be heard. This is why in space, where there is a vacuum, no one can hear you scream.
Sound waves travel at different speeds depending on the medium they move through.
The speed of sound is faster in materials that are more rigid and less compressible, such as solids, compared to liquids, and it is faster in liquids than in gases.
Factors like density and rigidity of the medium as well as temperature, affect the speed of sound.