More vocal, external, active participation, it seems to me:
Religious singing by the people is to be intelligently fostered so that in devotions and sacred exercises, as also during liturgical services, the voices of the faithful may ring out according to the norms and requirements of the rubrics.
I’m struck by the “intelligent” fostering of song. That implies planning, setting goals, working in stages, and actually catechizing people through music. This section also underscores the importance of the people participating as the rubrics indicate. In the Roman Missal, the rubrics are clear when the people sing, when they may sing, when the choir may sing alone. At no time in the Roman Rite is there a prescription in the liturgical text for the people to never sing or always listen. A music director unable to come to terms with this might find a happier home in the conservatory.
This isn’t to say the choir can never offer music exclusive of participation from the pew. But Vatican II’s direction from SC 28 was that people stick to their own role in the liturgy. There is time for musical performance and attendant listening at Mass–just when the rubrics say so.