Figured out what was causing the error sound when invoking the select synth dialogue under NVDA 2020.2 beta versions
I had the NV speech player add-on installed, and, now, when ran through disabling all non-essential add-ons, and then re-enabling them one at a time, this was the one that immediately started causing NVDA's error sound to occur when I hit NVDA + ctrl + S to bring up the select synthesizer dialogue.
When I then re-visited the following add-ons page, and, re-downloaded it, and, re-installed it, it didn't seem to be causing the issue anymore:
So, not sure if had an older version installed, or something else, but, just mentioning this here, in case.
"...resistance is futile...but, acceptance is versatile..."