Guten tag Origommi:
That's all the German I know, sad to say.
Need info about your car, please. Stock or aftermarket exhaust components? Please tell us if you have long tube headers (LT's), x- or h-pipe with or without catalysts (I suspect you have to have cats in Germany), and the cat-back.
If you have 3.5" dia. tips (aftermarket cat-back), you can get some Bassani baffles (about 8" or so long) that you can wrap with more fiberglass matting than they come with and this will quiet things down a lot. The ID of the baffles is 2.5" so they do not restrict the flow but they damp the sound. Sorry for the English/American units of measurement but you know how it is, lol. '
There is not much room to add bullet-type mufflers in series with the cat-back mufflers so you may need to go with "turbo" (baffled) mufflers rather than straight through.
Good luck with quieting down the car. What is the decibel limit in Germany? Don't Mustangs have to be TUV-tested or certified for import?
HTH,
Chris