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."

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

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."