I installed the M 4 Retro Drag slip on's on my 2013 14 R.
I really like the way they look and am happy with the sound.
I've been told by others riding with me that they aren't "too" loud.
Anyway, I couldn't find an audio clip on the site so I figured I'd post one up.
The bike is initially at fast, cold idle. At about 30 seconds into the clip it revs down to warm idle. I revved it to about 3000 rpm.
Hope this helps anyone considering buying the M 4 pipe.
Mike