Sound velocity in metals does change with tension or compression. For example, the slowing of sound velocity under tension is a significant factor in applications involving bolt elongation measurement. However I have not heard of this as a practical problem when measuring wall thickness of typical pressurized tanks or pipes, since the effect is fairly small under most conditions. Whether the velocity changes during a hydrostatic test would be of functional significance in your case would depend on the pressure being applied, the elastic constants of the metal in question, and especially the measurement accuracy required.