No Sound on a Mac
From X-Plane Wiki
If you have no sound output when running X-Plane on a Macintosh, read on.
For versions of OS X prior to 10.4, users needed to download the OpenAL drivers for OS X, available here. However, using X-Plane 9 requires at least version 10.4, so users running anything prior to that need to run the Apple software update before running X-Plane—this also serves to ensure that the OpenAL drivers are already installed.
Assuming version 10.4 or later is being used and the computer still puts out no sound, please see this OS X Knowledge Base article.