stereophonic (adj.)

before vowels stere-, word-forming element meaning "solid, firm; three-dimensional; stereophonic," from Greek stereos "solid" (from PIE root *ster- (1) "stiff").

"of or pertaining to sound, acoustic," 1793, from Greek phōnē "sound, voice" (from PIE root *bha- (2) "to speak, tell, say") + -ic.