Last Thursday, breaking news obliterated the time slot reserved for the scheduled episode of Soundtracks, and CNN didn’t find another place for it on the schedule all week. So tonight, perhaps optimistically (given the crushing speed of current events), CNN has announced plans to try, try again. And I’m hoping it gets shown, because I’ve been waiting for this documentary installment. It’s about the shooting of students at Kent State University by the Ohio National Guard in 1970. And musically, expect at least one unavoidable, unforgettable focal point: “Ohio,” the searing instant-response song by Crosby, Stills, Nash & Young.