What type of sound device is being used in the sentence, "Let freedom ring from every hill and molehill of Mississippi. From every mountainside, let freedom ring."

These lyrics are actually referring to a bell. Back in the day, much of the technology used nowadays wasn't created yet. A rhetorical bell, as they are implying here, is being used to say, "Listen to us. Listen to what we have to say."

Its not a sound its a substitute for the actual meaning its let freedom be heard