Fix airplay / shairport low volume output

[hn]

Shairplay does not use the hardware mixer (at least with my external Raspberry Pi DAC) and as a result the volume is very low.

This fix inserts the 'mixer_control_name' setting in shairport.conf. Notably, the 'mixer' variable is exported in airplay_emulation/index.js (line 150) but not yet used in the config file.

Please include this fix -- or at least a commented line (// mixer_control_name = ...) in the config file to give a hint on how to set the hardware mixer.

[ashthespy]

IIRC there was duplicate of this that was closed for some reason..

EDIT: Yep - #1879

[hn]

I'm not an volumio/linux soundsystem expert. But for my setup the volume is too low without the 'mixer_control_name' statement (volume adjustment works in principle, but overall volume is extremely low when streaming music from an iPhone).

//mixer_control_name = "${mixer}"; // necessary for some external DAC sound cards

I suggest to include the commented statement so that others may have a hint for this solution. There are other comments within the file, so one single additional comment wouldn't decrease readability.