What might be inside my speaker switch?
It has 1 and 2 in parallel with 3 and 4. Within each pair, it switches the speakers in one at a time or in series. There is a load resistor to keep the impedance at or below 220\$\Omega\$

I was curious enough to work this though, so I'm going to write an answer describing how I worked it out.

There are probably a number of ways you could wire something like this.  The first clue is the specification that the impedance presented to the amplifier would be no lower than half the speaker impedance.  This is a big clue that some of the time the speakers are in parallel. The second is that amplifiers put out quite a lot of power and this is a small box, so there is unlikely to be any resistors standing in for unused speakers or it would get very hot. 

Google turns up some photos where someone has taken one apart. That is a big help. Cropped, and with labels added based on the other photos, we have:

Two things are immediately obvious:


The left and right channel are laid out exactly the same, and don't connect to each other anywhere. This is what you'd expect.
The wiring of speakers 1 &amp; 2 is identical to the wiring of speakers 3 &amp; 4 (except the resistors - I'll come back to them later), and connected in parallel. That tells us the first part of how this works.


These together mean we only have to bother tracing 1/4 of the wiring, which saves some time. Tracing out the wiring gives:







simulate this circuit &ndash; Schematic created using CircuitLab

Where I've drawn it out in all four combinations of switch positions, and highlighted the wires which are carrying current to the speakers. So that is the second half of how it works.

So What are the resistors for? Well when both switches are off, there is not path from amp+ to amp-, and not all amplifiers are happy driving an open circuit. They provide a 220\$\Omega\$ path through both switches to prevent that.

Why don't you hear a large change in volume from speaker 2 when you switch out speaker 1? Well, that's another question altogether. It will depends a bit how the amplifier reacts to changes to the load impedance, but also the human ear/brain combo has a very non-linear perception of loudness.

Finally, I though I would note down the impedance the amplifier sees with different switch positions (assuming all speakers are 8\$\Omega\$):


Any one speaker: 8\$\Omega\$
Speakers 1 &amp; 2 or 3 &amp; 4: 16\$\Omega\$
Any other pair of speakers: 4\$\Omega\$
Any three speakers: 5.3\$\Omega\$
All four speakers: 8 \$\Omega\$