Volume sounds best when using a logarithmic scale

[karlvr]

This commit adds a cubic scale to the correct playback volume, which results in a more linear sounding volume response. You can confirm this by watching the slider on alsamixer, which now moves linearly where it didn’t before, as the alsamixer slider accounts for this logarithmic volume scale.

[3onyc]

If I recall correctly this depends on the sound card, for me it's currently a linear volume increase.

[Aragur]

Working perfectly for RaspberryPi (1st generation and 3rd generation)