I just spent almost a whole day going through everything suggested in the NO SYSTEM SOUNDS messages.

With absolutely 'no joy'! >:(

But here is what I found that got it to FINALLY work:
I am not an IT guy, so don't know any xml entry rules - I can just copy and paste

So what was missing from your info is: when editing the dll.xml file and adding the following:
<Launch.Addon>
    <Name>Captain Sim Sound</Name>
    <Disabled>False</Disabled>
    <ManualLoad>False</ManualLoad>
    <Path>Captain_Sim\bin\cs.sound.dll</Path>
</Launch.Addon>

it seems to be vital to end this with the following line
</SimBase.Document>

I found out about this after running tis fix-it file (which doesn't include P3Dv4 btw!)...
dll-xml-cs-sound-fix.exe
...and comparing the dll.xml from P3Dv3 with the one from P3Dv4
That's when I noticed that the </SimBase.Document> line was missing and since adding that it's all good now. It just took too long to get there. :(