Although we introduce this 3-D wave equation for air only, the expression also correctly describes the propagation of sound waves in ANY fluid medium. All one has to do is to use the correct speed of sound. For example underwater sound in the ocean also is well described by this wave equation. And since the non-bony parts of the human body are mostly composed of water, this wave equation also explains how sound waves travel in the human body. Whenever someone has an ``ultrasound" performed on them, the sound waves travel through their tissues according to this wave equation. Here the value of c will vary depending on what the sound is traveling through: skin, fat, muscle, etc.
(Picture of underwater sound.)
(Picture of biomedical ultrasound.)