Apple Mail (macOS Catalina) Missing Send Sound

When my Luna Display module is plugged into the mini-display port of my iMac (Retina 5K, 27-inch, Late 2014) running macOD Catalina 10.16.6, the default swoosh sound when sending mail goes away. Removal of the Luna Display module restores this sound. Has anybody else noticed this happening to them? This is the kind of think that many people might not even pay attention to but none-the-less it does happen to me.

Astropad technical support has been cooperative in trying to help me but they are not able to reproduce the problem. While not a show stopped it does bother me not hearing that sound to confirm that an Apple eMail has been sent.

Any takers on this one???

Check the “Sound Effects” tab of the Mac’s Sound preference pane. A pop-up there specifies where audio gets routed, and perhaps is set to an undesired ouput device.

My audio is properly routed to thru the headphone jack to an Altec Lansing external stereo speaker with a subwoofer and they work just fine. The Apply mail send sound is very clearly present UNLESS the LunaDisplay module is plugged into the Mini Display Port. The other system sounds are not effected. If my sound output had been misdirected it would act this way WITHOUT the LunaDisplay plugged in and most likely I would hear no sounds at all.

Any other suggest is welcome. Thanks.