> how does any audio signal get to the other side of the circuit??
The voltage across a cap changes "slowly".
How slow depends on the resistance in the circuit.
For audio, we usually pick a combination of R and C so that it takes most of one second for the cap voltage to change a lot.
So during a 20 cycle per second wave, the voltage changes very little.
Say you have an amplifier output that sits at 100V DC, and an input that has to sit at zero V.
Put a cap in between, with proportional resistors on each side.
When you turn-on, the "100VDC" side naturally starts from zero (dead) and rises to 100V DC as the tubes warm up, around 10 seconds.
The "zero V" side tends to rise, but the resistor on the zero V side "slowly" drains off the capacitor.
The capacitor ends up (after many seconds) with 100V across it: 100V on one side, zero V on the other side.
Now bump the "100V" side to 101V. Instantly the "zero V" side rises to 1V. And then it drains away, "slowly". But audio waves are always changing, back and forth, they don't stand still. If the capacitor-resistor combination is a good size for audio, you get a 1V jump that starts to drop to 0.9V but barely gets there in the time of a 20Hz wave. So all audio 20Hz and up is passed at 90%-99.9% of the original level, 1Hz (which we don't want) passes at about 10%, and DC is nearly zero. (It would be zero if we could wait for infinite time.)
> current does not go though a cap... electrons move
Capture an electron in a bottle and show it to me.
"Current" DOES "flow" in a capacitor. Electrons (if they really exist) don't cross the insulator, but so what? Trucks don't cross oceans, but you put a part on a truck in Japan, a week later you take that part off a truck in NJ, who cares about the truck?