Underworld river doesn't move audibly (4)
I believe the answer is:
styx
'underworld river' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for styx that I've seen before include "River separating Earth from Hades" , "Underground river" , "Underworld river ferried by Charon" , "Hell's river" , "River of hell" .)