Tavis Smiley and others who knew King at the time explore the significance of his Riverside Church address against the Vietnam War on April 4, 1967 in this PBD special, just aired
http://www.pbs.org/kcet/tavissmiley/reports/episode-two.html
Here is the audio and text of the actual speech in full:
http://www.americanrhetoric.com/speeches/mlkatimetobreaksilence.htm