I just tried a different saved airport/parked at the gate and all appears to be fine. I did have to turn the external speaker volume up fairly high though.
Doesn't happen here, and nobody else reported it before, except of course a couple of cases with 3rd party camera add-ons or very strange airports with non-standard altitude such Aerosoft Madeira, which doesn't work at all with GSX, not just the sound.
Munich obviously doesn't like me
There's nothing strange about Munich and it's probably the last airport where I would expect any problems, especially the default one.
Note that GSX supports 3D surround sound, which means all its sounds will be made by the speaker according to the correct position in space so, it's possible if your sound setup is configured to support surround sound, but you don't have the corresponding speakers connected, sounds might disappear depending on your position relative to the sound emitter.