Power transmitted by a wave on a string: $P = \frac{1}{2}\mu\omega^2A^2v$. For sound waves, intensity $I = P/A_{\text{area}} = \frac{1}{2}\rho v\omega^2 A^2 = p_0^2/(2\rho v)$ where $p_0$ is pressure amplitude. The pressure and displacement are 90 degrees out of phase in a sound wave: pressure nodes coincide with displacement antinodes and vice versa. This distinction is important for standing waves in pipes.