Move your GaugeSound.dll out of the Modules folder, and into your main FS9 directory.
Then delete the FS9.cfg entry for it under your [OldModules]
section.
Here is what my Modules folder, FS9 directory, and FS9.cfg [OldModules] sections look like:



I've never had any kind of errors running this, and the setup is the same in my Silver Wings, Golden Wings, Vanilla, and main installs of FS9.
Running XPpro SP3.
I'm also emailing you a copy of FSSound.dll version 1.6.0.15
Which I think is the newest version. Just in case that's what is causing your problem.

That fact that yours & Opa's are identical, yet you get the error, and that mine is nothing like either of yours, and I get no errors, more or less proves the "Mysteries of life" theory.
