Sound moves at a speed of approximately 1500 m/sec in sea water and moves faster with increasing temperature, salinity and pressure. Temperature is the dominant variable but when the temperature does not change much (at about 1000 meter) the pressure term dominates and at that point you get a sound speed minimum. Salinity does not have much of an effect.

Using the CTD, we measure the temperature, pressure (depth) and salinity (salinity is actually calculated from temperature, pressure and conductivity) and from these measurements the sound velocity is determined.
On this cruise, our measurements are only down to 800-900 meters so as you can see- the sound speed closely follows the temperature.