Final answer:
The speed of sound in a medium is determined by its rigidity and density. Hydrogen is less dense than air, which is why sound travels faster in hydrogen.
Step-by-step explanation:
The speed of sound in a medium is determined by a combination of the medium's rigidity (or compressibility in gases) and its density. The more rigid (or less compressible) the medium, the faster the speed of sound. The greater the density of a medium, the slower the speed of sound. Since hydrogen is less dense than air, sound travels faster in hydrogen than in air.