From the information available for the M5 it seems the Turbo cuts out both low and high frequencies in the exhaust note and again the induction noise is altered so you don't get as much growl. On top of that the interior cabin is very well insulated to prevent wind and road noise and again the low and high frequencies are what are lost first.
So what you hear in the car without the symposer is just the mid section of what a normally aspirated six would sound like with the windows down.
The sound symposer adds back the frequencies that the M department considers are lost by adding a Turbo and lots of sound insulation. A bit like Simon Cowell using autotune.