Vatican website translation:
46. Besides the commission on the sacred liturgy, every diocese, as far as possible, should have commissions for sacred music and sacred art.
These three commissions must work in closest collaboration; indeed it will often be best to fuse the three of them into one single commission.
46. Praeter Commissionem de sacra Liturgia, in quavis dioecesi constituantur, quantum fieri potest, etiam Commissiones de Musica sacra et de Arte sacra.
Necessarium est ut hae tres Commissiones consociatis viribus adlaborent; immo non raro congruum erit ut in unam Commissionem coalescant.
Slavishly literal translation:
46. In addition to the Commission on sacred Liturgy, in whatever diocese there may also be constituted, insofar as possible, Commissions on sacred Music and on sacred Art.
It is necessary that these three Commissions should work together with associated men [people]; indeed it will not rarely be appropriate that they should coalesce into one Commission.
The Council Fathers’ directives for promoting “pastoral liturgical action” conclude by calling for the development at diocesan level of commissions on sacred music and sacred art parallel to what they had decreed in art. 45 concerning diocesan commissions of sacred liturgy. They do note, however, that in order to guarantee that the work undertaken be properly coordinated it may be wise to structure these three areas of pastoral liturgical action in a single diocesan commission. They do not indicate the purview of these commissions, presumably trusting that such details are more appropriately dealt with by the local bishop.
Note that there is call for a further level of subsidiarity in promoting pastoral liturgical action by the development of parish or religious community commissions for sacred liturgy, music and/or art. However there is nothing forbidding such developments either. Pray Tell readers may wish to report: 1) on whether their diocesan structures have three commissions or have yoked the three areas into one commission and how effective that has made their work; and 2) what their experience is of parish or religious community liturgy commissions/committees: their membership, activities, formation, and effectiveness.