99 Since the divine office is the voice of the Church, that is of the whole mystical body publicly praising God, those clerics who are not obliged to office in choir, especially priests who live together or who assemble for any purpose, are urged to pray at least some part of the divine office in common. All who pray the divine office, whether in choir or in common, should fulfill the task entrusted to them as perfectly as possible: this refers not only to the internal devotion of their minds but also to their external manner of celebration. It is, moreover, fitting that the office, both in choir and in common, be sung when possible.
Source: Sacrosanctum Concilium (Vatican.va)