The Divine Command to sing is well-established in Sacred Scripture and Holy Tradition of the Church. Sometimes it is a fact that the whole assembly is not meant to sing. This may be because it is a technically advanced piece or that the notation and words are not available in advance to be distributed to all the people. Sometimes the song is proper to the priest only or the cantor or choir. In these cases, we do not sing. There is a high incidence of this in the Extraordinary form. However, in the OF and other liturgies such as the Byzantine Divine Liturgies, verbal participation by the assembly is rigorously encouraged and required. While the first and foremost mode of full, conscious and active participation is interior prayer and joining oneself the the liturgical action, full participation is further realized when the assembly takes part in the singing as prescribed by the Church. This is a time-honored tradition in the liturgical life of the Church.
The Psalms are primeval examples of liturgical corporate song. They were indisputably set to music and used as the hymnal of the Jewish Temple in antiquity. Their very words exhort all the people to sing praises to the LORD. The Psalms are the highest form of prayer ever known to mankind. They figure prominently in all liturgical worship in an unbroken tradition from Temple times to the present day with the Mass and the Liturgy of the Hours. The Church has always preferred song as the mode of liturgical celebration.
It is no accident that a recited Mass was known as a "Low Mass" because the spoken word is of a lower order than the sung or chanted word. With the Ordinary Form we have more options to mix-and-match song and recitation in the same liturgy, but we still have guidelines on which parts are so important that they should be rendered with music as often as possible. ProVobis has a point that hymns are optional. Hymns are the fourth and least preferred option for adding music to the liturgy. The Proper Antiphons are an integral and constitutive part of the Mass, and these should be sung by the choir or by all the people, in preference over and above mere hymns, but this guideline is more frequently observed in the breach.
It is abundantly clear to me that the Church requires us to sing just as God has commanded it in Holy Scripture from before the Church existed. Expressing prayer in the form of song is a sacred duty that we all have. We should all strive to practice and rehearse so that our prayer is a sacred offering to the LORD.