"In a sacred place only those things are to be permitted which serve to exercise or promote worship, piety and religion. Anything out of harmony with the holiness the place is forbidden. The Ordinary may, however, for individual cases, permit other uses, provided they are not contrary to the sacred character of the place."There is also a 1987 declaration by the Congregation for Divine Worship and the Discipline of the Sacraments on "Concerts in Churches".
Regarding the 99 names of Allah concert, the Cathedral authorities would argue that singing the names of Allah promotes peace and harmony and inter-religious dialogue etc. and is therefore OK. They have repeatedly made the point that this was not an act of worship but a concert.
The CDW's practical directives say that concerts should only be held in Church to perform music of a religious character and not music of a secular character, however beautiful. (It probably did not occur to the congregation to specify that the religion concerned should be the Christian one.)
One interesting provision is that:
In order that the sacred character of a church be conserved in the matter of concerts, the Ordinary can specify that:This brings up a more general question which needs a post to itself.
c. Entrance to the church must be without payment and open to all.