Sound coming through the mouth? Through the ear! (5)
I believe the answer is:
aural
'through the ear' is the definition.
(aural means relating to the ears)
'sound coming through the mouth?' is the wordplay.
'sound' indicates a 'sounds like' (homophone) clue.
'coming through the mouth?' becomes 'oral' (oral means relating to the mouth).
'oral' is a homophone of 'AURAL'.
(Other definitions for aural that I've seen before include "Relating to sense of hearing" , "Relating to the ear" , "Received by the ears" , "but not in the nose!" , "To do with an ear or hearing" .)