Leonardo DaVinci, the famous Italian thinker and artist, is usually credited with discovering that sound moves in waves.  He made this discovery around the year 1500.

However, some accounts say that the Roman philosopher Seneca actually discovered sound waves in the first century AD.

It is known for sure that later on another Italian, Galileo Galilei, discovered more of the properties of sound waves.  He discovered that the frequency of the sound wave is the factor that determines what pitch the sound will have.  He discovered this in or around the 1660s.