A new documentary, "Sound of the Police," traces the country's complex history of policing – from the formation of slave patrols to more recent acts of police brutality. WATCH NIGHTLINE EPISODES: abc.go.com/shows/nightline ALSO AVAILABLE ON HULU: hulu.tv/2wSmSrZ #nightline #soundofthepolice #documentary #policing #blm #blacklivesmatter #abcnews