Warm or hot air particles have more energy than that of cool air. As a result they move faster than cool air particles. Since sound propogation involves movement or vibration of particles, sound travels faster in warm air
Heat allows particles to gain more energy, including those in the air. This is why heating makes chemical reactions happen quicker. As the air particles have more energy, they will move faster and bump into each other faster, allowing for the sound to travel quicker
