I was recently comparing several powered speaker systems, but didn’t like the audible *Pop* I heard when plugging and unplugging the 3.5mm headphone jack into each of them. I made a switchbox that eliminates this pop by switching the one input (my mobile phone) to any of the four outputs. I didn’t have the right rotary switch at the time I built it, so it actually has two knobs that select where the input signal goes, meaning I can have two systems playing at the same time. Alternately, the box can be used to switch either of two inputs selected with one knob to either of two outputs selected with the second knob. Even more alternately, the box can be used to switch four inputs to a single output.
I few days later, after I purchased(!) the correct rotary switch, I built a second switch box. This one has stereo logarithmic-taper potentiometers wired as voltage dividers, effectively acting as independent volume knobs on each channel. With these volume controls, the playback levels of speaker systems with different gains can be matched, for ease of comparison.