Unvaried sound not found in satellite echo (8)
I believe the answer is:
monotone
'unvaried sound' is the definition.
(a sound or tone which doesn't vary)
'not found in satellite echo' is the wordplay.
'found in' indicates putting letters inside.
'satellite' becomes 'moon' (moon is a kind of satellite).
'echo' becomes 'e' (phonetic alphabet: alpha, bravo, charlie etc.).
'not' put inside 'moon' is 'monoton'.
'monoton'+'e'='MONOTONE'
(Other definitions for monotone that I've seen before include "Flat and unchanging in pitch" , "Flat voice" , "Single unvaried sound level" , "Utterance at a single unvaried level" , "Sound of unchanging pitch" .)