Sound transmission through two or more speakers (6)
I believe the answer is:
stereo
'sound transmission through two or more speakers' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for stereo that I've seen before include "System of sound reproduction" , "Tape recorder" , "System that gives a three-dimensional sound" , "Three dimensional sound effect" , "Three-dimensional sound effect, in short" .)