Sound speed - also called sound velocity - can be measured directly using a sound velocimeter or it can be calculated indirectly from salinity temperature and depth measurements.

This article compares the measurement of sound velocity through a CTD (indirect) and a sound velocimeter (direct).  It discusses issues with CTD's including spiking and accuracy.

This discussion paper identifies some of the data errors that can occur in CTD derived data - salinity, density, and sound velocity - when instruments with low sampling rates, slow sensor response times, or mismatched sensor response times are used.