Did you accept cs.sound.dll as trusted? It should appear during the first run of FSX after installing the 777.

Check to see if you have this entry in DLL.xml:

<Launch.Addon>
     <Name>Captain Sim 777 Sound</Name>
     <Disabled>False</Disabled>
     <Path>Captain_Sim\777\cs.sound.dll</Path>
</Launch.Addon>

Open the DLL.xml by going to Start > Run > enter %AppData%\Microsoft\FSX\DLL.xml > OK

If you have not accepted it, it might be best to uninstall the 777, then reinstall it, just to make sure that it asks you to accept the cs.sound.dll when you start FSX.