That makes me think that the circuit is:

audio_generating_source - conductor wire A - destination device -
conductor
wire B - earth.


Which it is not.

When I think it needs to be:

audio_generating_source - conductor wire A - destination device -
conductor
wire B - audio_generating_source


That's what it is, and conductor wire B is the outer shield of the cable,
which is called "ground."

That is an ordinary (not differential) signal. A differential signal would
use 2 wires neither of which is grounded in any of the senses of the world.

OK, they why ground? Maybe I just don't get the whole point of grounding
if it
makes no difference on the audio signal once they are differenced against
each other.

For the sake of shielding, we arrange it so that all of the inputs and
outputs of an amplifier have their outer shields connected together and to
the chassis of the amplifier. That keeps hum from being picked up. Every
AC connection still has two conductors (inner pin and outer shield).

For the sake of high-voltage safety, we arrange it so that the chassis of
the amplifier *may* also be connected to the earth through the power lines.
Not all amplifiers do this. Laboratory test equipment does.

That is the connection between the three senses of "ground" that I defined 2
messages back.

Audio systems do not have, or require, rods driven into the ground. Let go
of that concept. That is not what "ground" means with an audio signal. Let
go of that.