Answer
Helium molecules move faster than air molecules (composed mostly of nitrogen and oxygen) .
Work Step by Step
In a gas, the molecules have the same temperature and average kinetic energy, $\frac{1}{2} mv^{2}$. Lighter helium atoms (smaller m) are moving faster (higher v) than the more massive diatomic nitrogen and oxygen molecules.
Because helium atoms move faster, they bump into each other more frequently compared to air molecules, meaning a sound wave (which is transmitted via bumping between particles in the medium) travels more rapidly.
