On the etymology of "Stereophonic"
Solution 1:
Stereo in stereophonic refers to the three-dimensional sense:
Stereophonic sound or, more commonly, stereo, is a method of sound reproduction that creates an illusion of multi-directional audible perspective. (Wikipedia)
Stereo (etymology)
before vowels stere-, word-forming element meaning "solid, firm; three-dimensional; stereophonic," from Greek stereos "solid". (Etymonline)