Open the dll.xml file (enter %APPDATA%\Microsoft\FSX\DLL.XML in the run command to open it) and see if it has the following entries for the Captain Sim 767 in it:

 <Launch.Addon>
   <Name>Captain Sim 767 Menu</Name>
   <Disabled>False</Disabled>
   <Path>Captain_Sim\Captain_Sim.b767.menu.dll</Path>
 </Launch.Addon>
 <Launch.Addon>
   <Name>Captain Sim 767 Sound</Name>
   <Disabled>False</Disabled>
   <Path>Captain_Sim\767\cs.sound.dll</Path>
 </Launch.Addon>

The second one is the one we are really interested in because it enables the 767 cs.sounds.dll to work.

Seeing as you were asked about allowing the cs.sound.dll when you first start FSX (after installing the 767), I suspect it is in there. So something else is going on here, but I'm not sure what. :(

Mark