34. Since the celebration of Mass by its nature has a “communitarian” character,[Sacrosanctum Concilium 26-27; Eucharisticum Mysterium 3d] both the dialogues between the Priest and the assembled faithful, and the acclamations are of great significance;[SC 30] for they are not simply outward signs of communal celebration but foster and bring about communion between Priest and people.
35. The acclamations and the responses of the faithful to the Priest’s greetings and prayers constitute that level of active participation that is to be made by the assembled faithful in every form of the Mass, so that the action of the whole community may be clearly expressed and fostered.[Musicam Sacram 16a]
What do these sections tell us about the Mass? Note the importance of the dialogues and acclamations for expressing “outward” community, but also a “communion” between the priest and the people. Do these elements of the Mass in our parishes contribute to this? Does singing the dialogues always foster priest-assembly unity?
Section 35 is important. Acclamations and responses (note the change in word order) are to be done–preferably sung (note the reference document) in every form of the Mass. This is not legislation suggesting that the people always sing (or speak) every acclamation. But as a matter of standard practice, the people would be expected to outwardly voice those elements assigned to them in the rubrics of the Order of Mass. These would include acclamations: after each of the readings, at the Gospel, and during the Eucharistic Prayer.
I would think that alternate forms (choir-only music at a time of acclamation) would need to be done in such a way that the worship “of the whole community may be clearly expressed and fostered.” Speaking for myself, choir-only music for the acclamations is possible. But it would need, in my mind, to satisfy the greater good, namely the entire community expressing an act of worship to God.