Narrow inlet of the sea (5)
(Other definitions for sound that I've seen before include "note, for instance" , "Not diseased or damaged - listen" , "Financially stable" , "Unimpaired" , "to do the plumbing?" .)
(Other definitions for firth that I've seen before include "Estuary" , "Arm of the sea - actor Colin" , "Scottish sea-inlet" , "Narrow sea inlet in Scotland" , "Scottish river mouth" .)