In the letter saying I had to get a "justificatifs concernant le niveau sonore", the text "Brigade de police de LYON" was scribbled on the side. I showed the paper to a French person who has been very helpful in formal matters, and was given the advice to try calling the Annecy police, since they are closer. A quick call confirmed this, and I got an appointment for Saturday 16:00 (sic!).

Driving to Annecy took me slightly less than one hour, but because of the horrendously difficult one-way system of the town center, I barely got to the police station in time. Going in, I presented my case, and got a kind of strange look back. The officer called someone (presumably some garage), and they told him they hadn't heard anything about the appointment. I continued insisting that I had talked to somebody at the Annecy police station, and repeated the date and time. The the officer asked me when exactly I had made the call, and told me to wait a few minutes.

When the officer got back to me, I was told that the secretary had been misinformed: They had stopped doing this kind of tests two years ago! I would have to call the Lyon police station on Monday. So I basically drove for about three hours for nothing!

A small afterword: When calling the Lyon police today, the officer didn't understand me. When I started to explain the third time, he just hung up...