Reason for upper or lower case for symbolic values is just because we are trying to keep MMA case insensitive.
Note, also, that more than one symbolic value is not standard music: 'off' and 'm'.
Problem/error with the code is that it accepts things like '80+20%' but doesn't do anything with it ... accepts it as 80. 'FF-20%' is flagged as an error. Can you fix that.
I have gone though my 1000+ files can fixed all the volume settings without the % signs. Damn, I didn't know there were so many
Working on the docs right now.
Best,