Answer
The sound intensity decreases by a factor of 9.
Work Step by Step
The inverse-square law predicts that the intensity would be $\frac{1}{9}$ its original value if the distance to the source were tripled.
The outgoing sound power is distributed over the surface of a sphere with 9 times the area of the original sphere, so the received power per unit area (intensity) is one-ninth of its previous value.