Re: What electrically travels through a home theater audio cable?

Good question!

The answer lies in the electronics of the CD player:  the digital signal 
encoded on the compact disk is sampled at 44.1 KHz (although some high-
end CD players use higher sampling frequencies) and converted to the 20Hz-
-20KHz analog audio signal.  This sampling and conversion is necessary as 
the information on the CD is in digital form and inaudible or, if you 
could hear it, would sound like garbage.  The conversion is done using a 
circuit called a "digital-to-analog converter", also known as a DAC, D2A, 
or D/A.  This DAC converts the digital signal on the CD to an analog 
signal, which forms the output of the CD player.  Then the analog signal 
output goes to the preamp, and the output current returns via the shield 
of the RCA cable.

I hope this helps!