The speed of ultrasound is depend:

Ultrasonic testing on metals, plastics, composites, and ceramics is common in industrial applications. Wood and paper products are the only common engineering materials that are not suitable for ultrasonic testing with conventional equipment.

The speed of US in tissue is affected by stiffness and density. Stiffer materials, such as solid, transmit ultrasound more quickly. Ultrasound can be transmitted, attenuated, or reflected as it enters tissue.