Only a single fluid exists at the liquid-vapor critical point; it exhibits properties of both phases.
In gases like air, sound travels via collisions with the molecules; sound travels faster in gases when the temperature is increased (and the density is held constant).
Sound travels faster in air as the temperature increases.
The speed of sound depends on the phase of the material (solid, liquid, or gas). In general, sound travels fastest in solids, then liquids, then gases.