I found the cause of my problem:

On MacOS, the "Support Files" referenced in some of the acoustics examples are installed inside of Mathematica's "application bundle":

/Applications/Mathematica.app/Contents/SystemFiles/Components/PDEModels/SupportFiles

The code in the "Eigenfrequencies of a Room" example looks for SupportFiles under

/Library/Wolfram/Documentation/13.2/en-us/Documentation/

a directory which exists, but is not the location for SupportFiles on a MacOS installation. Editing the code to point to the correct path fixes the problem. Annoyingly, when built-in examples are loaded in Mathematica, the "Save As..." option seems to be disabled (I can understand "Save" being disabled so you don't overwrite internal documentation, which should be read-only). I just made a copy of the notebook under my user directory and it works great after fixing the path.

Another notebook in the "Acoustic Models" series (for an automotive muffler) contains the correct path.

Hopefully this workaround will be helpful if anyone tries experimenting with this on a Mac. Files inside the application bundle do not show up in a normal system search on MacOS, so are harder to find. I have no idea if the paths resolve correctly on Windows or Linux versions of Mathematica.