Even better: a volume application that measures system sound and auto-mixes output audio to fall within set tolerances around the average decibel reading of that sound.

not just systemsound but also background noise. if i am listening to an audiobook while out in the street, i frequently have to adjust the volume when the street noise gets to much.

unlike listening to music where missing some lyrics might not bother you, for a book every word counts, and if i miss something because a noisy truck just passed by then i have to skip back and repeat.