Sound element is not often used in Little Alchemy 2, and it could be used to combine Alarm Clock, Avalanche, Bell, and Stethoscope. Making Sound is not a complicated process, just like making the element Wave, you need to make Cloud, Storm, and Lake elements first. Check out the detailed walkthrough and try it yourself.
Earth + Earth = Land
Earth + Land = Continent
Continent + Continent = Planet
Air + Planet = Atmosphere
Water + Atmosphere = Cloud
Cloud + Cloud = Storm
Water + Water = Puddle
Water + Puddle = Pond
Water + Pond = Lake
Storm + Lake = Wave
Air + Wave = Sound