I remember it well. Not only was the talk scathing, a list got passed around listing all the bands/artists that the Bethelites had to discard during the cleaning out they had.
I don't know if the speaker initiated the passing of the list or if it was some over-righteous music hater in the mob audience but it was a doozy of a list. One column listed the offending band/artist and a second column listed the offense. Included were not just the "bad" bands of the day like Zeppelin, Sabbath, etc. but the more middle of the road stuff was targeted too. The one I remember standing out the most was James Taylor: he spent time in a mental institution. (OH MY GOD!! THE HORROR!)
The assembly the week after ours went one better and played a couple of the "backward masking" songs backwards to show the audience the evil influence of the debasing music.
I can't believe that I actually threw out albums that I owned based on that talk. What an idiot I was. Well, I've more than made up for it since. I got some good shit playing on the stereo these days.