In general for all formats including music, use no more of the original than is required to meet your learning objectives. In some cases, use of the complete musical recordings, scores, or other publications may be considered fair use if it is required to meet the learning objectives.
As with all fair use determinations, the question of tranformative use is key. In the case of music education, the original purpose of a song or composition is typically entertainment or artistic creativity; the course related transformative purpose in using the song/composition is teaching/learning, scholarship, or commentary (Best Practices 2010).
Faculty should always balance the amount copied from the original work with the amount needed to achieve the learning objectives.
General guidelines with regard to use of copyrighted musical materials for instruction: