Relating to sound or hearing (8)
I believe the answer is:
acoustic
I believe this is a double definition.
'relating to sound' is the first definition.
(I've seen this before)
'hearing' is the second definition.
(I've seen this before)
(Other definitions for acoustic that I've seen before include "dispensing with amps?" , "Pertaining to hearing" , "Of the physical properties of sound" , "Like sound" , "Of sound or the sense of hearing" .)