Complex sentence: In addition, a dance instructor not familiar with the exact musical rhythms should not demonstrate these dance rhythms without music: better use a slowed-down playback, lest the dancers might become confused at full speed.
Split into simpler sentences: In addition, a dance instructor not familiar with the exact musical rhythms should not demonstrate these dance rhythms without music. It would be best to use a slowed-down playback, lest the dancers become confused at full speed.