Yale DOES give specific guidance. They HIGHLY recommend ONLY two LoRs.
The 2 required LORs for Yale are from core academic teachers of your choosing. Choose wisely teachers who can write intimately about your presence in the classroom. That’s what LoRs (for Yale) are for. If you choose to submit a Music supplement, plan on NOT violating Yale’s suggestion.
If, you feel that a single music-related LoR would be enormously helpful to your package, take the risk and send it as your 3rd supplemental LoR.