Prelim Practice: Sound Waves versus Waves on a String

The following equation is true for strings:

$$-\frac{\partial}{\partial x} \left[ \tau\frac{\partial y}{\... ...u{\left(\frac{\partial y}{\partial t}\right)}^2 \right] = 0\;.$$ (1)

It expresses conservation of energy for waves propagating on a string.

What equation expresses conservation of energy for sound waves?
Hint: Use the unified picture for string and sound waves presented in class and take a good guess!