Here is a semi-interior (my driver door was open) video from my Eisenmann race exhaust I had installed on my M6. Unfortunately, I didn't rev the engine much at all as my engine was not up to proper temperature, so this is not a fair comparison. However, you can at least get a taste of the sound characteristics. There are certainly MUCH better videos out there.
Below is a video of my RPi GTM Race Exhaust. When I shot this video, I had learned from my past mistake of not revving high enough. So, you can REALLY hear this one.
Again, not a fair comparison due to the lack of revving on the Eisenmann.