The speed at which sound travels in the ocean is affected by temperature, salinity, and pressure. Sound travels faster with increasing temperature, salinity, and pressure (See How fast does sound travel?). In the deep, open ocean at mid-latitudes, roughly half-way between the equator and the North or South poles, water temperature decreases with depth, salinity can either increase or decrease with depth, and pressure always increases with depth.
There is a minimum in sound speed at a depth of roughly 1000 m in mid-latitudes. Sound speed near the surface decreases with increasing depth due to decreasing temperature. The sound speed at the surface is fast because the ocean is warmed by the sun heating the upper layers of the ocean. As the depth increases, the water temperature gets colder and colder until it reaches a nearly constant value of about 2°C for depths below roughly 1000 m. Where temperature is nearly constant, the pressure of the water has the largest effect on sound speed. Because pressure increases with depth, sound speed increases with depth. Salinity has a much smaller effect on sound speed than temperature or pressure at most locations in the ocean. This is because the effect of salinity on sound speed is small and salinity changes in the open ocean are also small.
The result is that sound travels slowest at a depth of roughly 1000 meters in mid-latitudes. This minimum in sound speed forms a sound channel or waveguide through which sound can propagate over long distances.