It would be useful to be able to delegate Volumio’s volume control somewhere besides upstream “Software”, e.g. ALSA, or output device “Hardware”, e.g. an I2C DAC. An “External” (or “Plugin”, or some such) mixer type option that (unlike “None”) kept the volume slider and associated state change notifications active but did nothing else, would allow volume control by other hardware, using an associated plugin to monitor the volume setting.
One application might be multi-channel attenuation between active crossovers and power amplification:
<volumio> -> <output device> -> (stereo signal) -> <crossover> -> (multi-channel signal) -> <gain control circuit> -> <power amplifiers>
Where the gain control circuit handles volume control via a Volumio plugin.
It is possible to get the desired effect in Volumio3 (3.832) by putting a bogus mixer control in the alsa_controller config file, so presumably it wouldn’t be too complicated to do it correctly in volumio code.
I posted a question on how to do this properly and was advised that it isn’t supported as of now (see Downstream Volume Control). It seems like a simple and useful feature; please consider it requested. Thanks!