Answer
The inverse-square relationship is no longer strictly true.
Work Step by Step
If the distance between the sound emitter and the sound receiver is doubled, the inverse square law predicts that the sound intensity $I$ is reduced by exactly a factor of 4 (equation 15.26).
In reality, energy absorption by air means that for a given source power and receiver distance, the received sound will be (slightly) weaker than that predicted by the ideal inverse-square law.