I suspect we are talking about different things. I am referring to the sheet of thick vinyl-covered cardboard that has a single central speakers attached to its underside in a P5 Mk II coupe. Installing stereo speakers involves no structural alteration to metal bits.
Even the most vindictive of car examiners could not possibly believe that making holes in a cardboard trim is a safety risk.
The comment was meant as a general reference for anyone considering fitting rear speakers to the rear parcel shelf in Victoria, Oz. I have no idea of the laws in other states or countries regarding fitting speakers to the rear shelf.