Add volume control

[jcard0na]

Add control commands to control volume.

[Ekolide]

Hi! I have have field-tested the Haxophone with some guitar pedals I think this is very much worth implementing. The standard volume is just too much for my pedalboard, overloading basically every input. There's input attenuation on my digital effects pedal, but even at it's max setting of -12dB, it's still too hot and regularly distorts on the input.

And two other considerations:

• Speaking of connecting the Haxophone to pedalboards, the MAX98357A documentation states that the output is a speaker output, and should be used to drive a speaker directly.

"All the above means that you can't connect the output into another amplifier, it should drive the speakers directly" (Adafruit, p. 6)

Attenuation through control commands make sense.

• Headroom/Polyphony. If the audio is always close to peak, there will be no headroom left for polyphonic modes, other effects, or to play multiple sounds without prior configuration. Since this might push the digital output to clip, the potential solution here might rather be to set the FluidSynth gain to something lower than 1, leaving some headroom.

Having control commands for the FluidSynth gain is one possibility. But is it possible to control the power supplied to the audio amplifier, or does this require hardware modifications? This could have the benefit of letting the user disable the power to the headphone out, for example if they are using the Haxophone as a MIDI instrument, simply hacking/coding, or otherwise want to conserve power.