I can not say what OrbiterSound exactly triggers on (as I have no insight of course), so whether it's really
clbkFocusChanged is pure speculation on my side
And because I still can not see what I'm really doing wrong here, I've pushed the source code to github to let "another pair of eyes" look onto it.
I initially wrote that code for Orbiter 2010 where it worked. Than came Orbiter 2016 and I couldn't get the same code to behave as expected anymore (regarding single vessel startup). I discussed that issue a while ago (can't find that thread again however).0
After some time I forgot about that add-on, but now when there's a 5.0 Version on the horizon it came back
---------- Post added at 15:50 ---------- Previous post was at 15:46 ----------
Side-Note (must not be relevant to this issue):
There was an issue with "Playback mode" and OrbiterSound, where the initial
clbkFocusChanged call was not done by Orbiter core,
but that was fixed in D3D9Client.
---------- Post added at 16:09 ---------- Previous post was at 15:50 ----------
Side-Side-Note:
I also would like to see pull requests from voice-actors increasing the quality of the "voices"
Some phrases like "decimal" or "MHz" are rare on free sound libraries in the web

Adding more voices ("French" and "Russian" for example might be nice) is easy: Just add a folder with the samples in the Sound/ComMFD folder.