Guess it’s time to make a ProffieOS 6.8.
(And maybe a 5.10, this bug is old!)
I’ve created a ProffieOS 6.8 zip file. It would be nice if someone could test it before I make it official.
The dynamic mixer fix should be the ONLY difference.
It can be downloaded here:
Well done. Nice that you’re versed in the way things work as much as you are @DeFi.
Very helpful, please stick around!
I see you wanted to go double safe and incorporated the datatype change for vol_ and the calculation change into 6.8. However, you changed vol_ += abs(v); to vol_ += v; which will not work, since v can and should go negative and consequently harm the square root. Putting this on a board will turn all sounds into pure scratchiness. Also, the file needs some tabs-to-spaces and indentation cleanup. I attached my working version of the file.
dynamic_mixer.h (5.3 KB)
Verifying this on an additional proffieboard, besides mine would be advisable.
oopsie 
Fixing.
See, this is exactly why I want people to check my work.
Thanks for reporting, helping me find it and checking my work!
It may sound trite, but working together makes everything better.
Updated git master, cherry-picked to v6.x branch, v6.8 tag updated and new zip file created and uploaded.
How about now?
Looking good apart from the remaining tabs/spaces and indentation inconsistencies, but that may be due to git and several diff tools ignoring whitespace changes.
gn8, it is already quite late here in Germany. 
Git does wacky things to indentions in my experience.
I always wonder …Did he really mean to do that? But I know better.