The sound [[Speed]] is a property of some [[Fluid]] that dictates the speed of interaction between particles in that fluid. Lower [[Density]] [[Fluid|Fluids]] such as air have lower [[Sound Speed]] than denser [[Fluid|Fluids]] such as water because for an interaction / [[Wave]] to propogate, each particle must hit another. Lower [[Density]] means each particle has to travel farther to hit another.