May a Non-Catholic Sing at Mass?

In the Church’s Directory for the Application of Principles and Norms on Ecumenism, this issue isn’t directly addressed. But based on the Directory’s provisions for non-Catholic readers at Mass, we think an inference can be made for a non-Catholic cantor or other singer:

The reading of Scripture during a Eucharistic celebration in the Catholic Church is to be done by members of that Church. On exceptional occasions and for a just cause, the Bishop of the diocese may permit a member of another Church or ecclesial Community to take on the task of reader (no. 133).

For similar reasons, including ecumenical ones, a bishop may allow—including through power deputed to his parish priests—a non-Catholic’s singing in such circumstances.