Why Radio waves are electromagnetic waves, the same phenomenon as visible light, infrared and X-rays, differing only in frequency. In a vacuum all electromagnetic waves travel at about 300,000,000 meters per second (3 x 10^8 m/s), the speed of light, usually written c. That constant is what lets you convert wavelength to frequency: wavelength in meters = 300 divided by frequency in MHz.
Watch out The speed of sound applies to mechanical pressure waves in air, roughly 343 m/s, about a million times slower; sound needs a medium, radio does not. A velocity factor less than 1, such as 0.86, describes a wave slowed down in a cable or other material, not in free space.
Radio is light you cannot see: 3 x 10^8 m/s, and 300 / MHz = wavelength in meters.