Do anybody know the purpose of these vents? There are from the bottom in the rear fender area in the trunk. (photo not mine)

 

If you close a door or the trunk, air pressure has to go somewhere. It is released at the back of the car, there are louvres mounted.

Your ears love that they are there.

 

It is to let air move behind the panels to help cool modules and amplifier that is behind the panels

 

Where the louvres exit the car?

 

My first thought was to let exhaust sound in the cabin... So we have now at least three theories (might be true all)

 

This is correct. Almost all modern cars have this somewhere in the boot/trunk and it allows venting of cabin pressure. It usually vents between the interior trim panel into the gap between the exterior sheet metal. This is the only function of it.

 

I'm pretty sure that with that panel removed you can see the vents, but I'm not taking mine apart to get a picture. Not today, anyway

There is one on each side, low at the very back of the compartment, exposed when the panel is removed. There are rubber flaps that act as one-way valves to let air out but keep debris out. They wouldn't stop water coming in if you were submerged, but you'd have bigger problems at that point.