Add volume control

[bytesnake]

This is a nice have, but (at least for me) not really useful. The sound volume can already be controlled in the system and you don't need a seperate control element. Perhaps if some songs are not equal equalised.

[mainrs]

Wouldn't implementing ReplayGain make more sense? Or just have people that are interested in equal volume levels include the ReplayGain metadata themselves.

[bytesnake]

Hey, this may not be needed, because the audio data is equalised to the full 16bit audio range anyway. (here)

Though this is problematic when the audio is very quiet except a single peak. We could introduce the inverse of the RMS value for the whole song as a gain, but the reference is still missing. The sound reproduction is clearly louder with 36 loudspeaker instead of just two. So the equalisation should act directly after the decoding step and apply a calculated gain, corresponding to the approximation of N non coherent loudspeaker. (with the result of an equalised perceived loudness)

[bytesnake]

#22