In Unity’s Project view, click on all six of the files in UnityProject\Assets\Plugins\x86\Q3DAudio and UnityProject\Assets\Plugins\x86_64\Q3DAudio and make sure they have the following Inspector traits:
Any Platform = checked (this was already correct for me)
Exclude Platforms: Android is checked (but Editor and Standalone are unchecked; they were already correct for me)
OS = Windows (not “Any OS”)
CPU = x86 or x86_64, depending on the subdirectory (this was already correct for me)