This is what is shown in the P3Dv5 dll.xml:

<?xml version="1.0" encoding="UTF-8"?>

<SimBase.Document Type="AceXML" version="3,0" id="dll">
  <Launch.Addon>
    <Name>Captain Sim Sound</Name>
    <Disabled>False</Disabled>
    <ManualLoad>False</ManualLoad>
    <Path>Captain_Sim\bin\cs.sound.dll</Path>
  </Launch.Addon>
</SimBase.Document>

Although what I find interesting, is that when I pasted the following:

<?xml version="1.0" encoding="Windows-1252"?>
<SimBase.Document Type="Launch" version="1,0">
<Launch.Addon>
    <Name>Captain Sim Sound</Name>
    <Disabled>False</Disabled>
    <ManualLoad>False</ManualLoad>
    <Path>Captain_Sim\bin\cs.sound.dll</Path>
  </Launch.Addon>
</SimBase.Document>

from the P3Dv4 dll.xml, that didn't work, so I removed the
<?xml version="1.0" encoding="Windows-1252"?>
<SimBase.Document Type="Launch" version="1,0">
</SimBase.Document>
elements of the dll.xml, and when I started P3Dv5, the sounds for the 757 III were working. I looked at the dll.xml again, and it had added the correct <?xml version etc...