Headphone volume doesn't change

[Claus Näveke]

First of all: Thx for this awesome lib!

I installed it on my AI-Thinker audio board and it works good, but the volume controll for the headphone doesn't seem to work. I use the example code and volume between 0 and 50 doesn't make any noticeable difference (it's all way to loud).

[Yveaux]

Thanks for the fix!