Currently 'mute' is only implemented if your Squeezeslave/SB Player is routed through a controlled amplifier eg Denon or similar. The Vol up/down issue in #21 is almost certainly because media_config has an incorrect value for AudioDev in the line that is for that room. Look for a typo or incorrect value in AudioDev.
All the best
but there are definitely no typos in it and the room number and device number are defintelly right.
So room is 4
The squeezebox is device 210
there are NO other device in the room
Room=4; AudioDev=210; VideoDev=; KeepScreenOnForOSD=no
should be all, right?