How does music connect us with God? Why are some forms inappropriate in some churches while others accept them?
Music has always played an important role in our liturgical celebrations. Our song is our prayer. It unites our breath, our bodies, our minds, our hearts and our souls in the worship of our God. Many great compositions throughout history were based on the Mass or even the liturgical year. Numerous composers, such as Haydn and Schubert - to name but two - were involved actively in the liturgy.
In 1963, chapter 6 of the Vatican II document Sacrosanctum concilium, entitled "Musica sacra" outlined the basic principles for music in the liturgy whose reform this same constitution mandated. It reiterated Pope Pius Xs position that music was to serve the liturgy and form an integral part of it. Interestingly, the document affirmed the priority of Latin but allowed the use of the vernacular as well. Over the years since the council, there has been a great development in the music which is used in our worship. For instance, we now hear more of "liturgical music", which underscores its indispensable role in our liturgy. It reveals the divine and enables the congregation to draw closer to God in ways unique to this art. This really is what makes any form of sacred music appropriate for use in our community prayer: it must connect us with our God.
Looking back over a 1500 year tradition, we see that it is rich and filled with so many varied, but valuable forms: chant, hymnody, the polyphonic tradition, the chorale tradition and more contemporary liturgical "songs". So, to answer the last part of your question, what music is considered appropriate for us? It must be God-centered. It can be old, it can be new. But it can only be used if it is worship - inspired by Christs teaching, uniting our voices in the community's prayer.