As relayed by the musicasacra.com/forum:
Dubium: A Major Catholic Basilica in America has the following practice: as the priest processes to the altar, they sing an opening hymn. Then, as the Bishop incenses the altar, they sing the Proper Introit in Gregorian chant. Is this practice licit?
Responsum (Feb 14, 2012 at 7:29 AM): Thank you for your question. Why would you think it illicit to sing both a hymn and the antiphon during the entrance, especially at a more solemn liturgy involving the bishop when the procession and the incensation of the altar might require more music to accompany the action? This is, in fact, the practice in many cathedrals, especially at stational masses of the bishop which include the whole presbyterate of a diocese (such as the Chrism Mass or ordinations).
Executive Director, Secretariat of Divine Worship, USCCB, 3211 4th St. NE, Washington, DC 20017
Dubium: What is the proper response to people who point out that the current GIRM does not allow this practice?
Responsum (Feb 14, 2012 at 9:58 AM): To say “the GIRM does not allow for this practice” is a bit of a stretch, because it simply gives several options for what could be sung at the entrance. It does not speak in one way or another about whether one could do both, because it speaks only to the normative practice of an entrance procession that includes priest, deacon, and other assisting ministers, and not a more elaborate entrance procession at a stational mass with the bishop and the presbyterate. The GIRM never speaks to every possible scenario that could take place.
Executive Director, Secretariat of Divine Worship, United States Conference of Catholic Bishops, 3211 4th St. NE, Washington, DC 20017