Simple question, or is it...

OK, you're beyond me. It has to do with flow of energy in EM field or something but I don't see how it has anything to do with the question of what's an audio signal and what's it travel through. 

Electric current in the cable is a flow of electric charge.  Exactly the same amount of electric charge that leaves the source comes back to it.  Energy is delivered from source to load by electromagnetic field in between wires.  It is called Poynting field and direction of this electromagnetic wave can be determined by Poynting vector.  Presence of the load creates voltage drop - an electric field between wires.  Current in the wire creates circular magnetic field around it.  These electric and magnetic fields are perpendicular.  If you imagine them as X-Y axis then Poynting vector will be perpendicular to them as "Z" axis.  This vector points always in the same direction, (from source to load) even with ac current, since both electric and magnetic fields change direction at the same time.  In the coax cable, that was mentioned, whole energy (electromagnetic field) flows inside thru dielectric between wire and the shield.

OK but when you aren't using coax cable the energy doesn't flow through the dielectric? 

When you use just plain lamp cord, energy flows between wires. It is strictly not between wires since electric fields bends, but also outside of the cable in-between planes like picture below shows:

https://www.google.com/url?sa=i&url=http%3A%2F%2Fwww.vias.org%2Falbert_ecomm%2Faec06_transmissio...