Pope John Paul II is certainly aware that the primary musical instrument of the liturgy is the human voice. That said, other musical instruments are used to accompany singers or to stand on their own as part of an effort of spiritual edification.
The pipe organ (not the electronic organ or the synthesizer) is mentioned first:
14. Again at the practical level, the Motu Proprio whose centenary it is also deals with the question of the musical instruments to be used in the Latin Liturgy. Among these, it recognizes without hesitation the prevalence of the pipe organ and establishes appropriate norms for its use[TlS 15-18]. The Second Vatican Council fully accepted my holy Predecessor’s approach, decreeing: “The pipe organ is to be held in high esteem in the Latin Church, for it is the traditional musical instrument, the sound of which can add a wonderful splendor to the Church’s ceremonies and powerfully lifts up people’s minds to God and to higher things”[SC120].
There is a pragmatic side to this favor. Organs really have no single parallel in terms of one instrument being able to lead a singing congregation. As for the reasoning in SC 120, there’s nothing to suggest that the individual wind instruments imitated by the organ cannot provide “splendor” or “power” or the uplifting of minds.
Nonetheless, it should be noted that contemporary compositions often use a diversity of musical forms that have a certain dignity of their own. To the extent that they are helpful to the prayer of the Church they can prove a precious enrichment. Care must be taken, however, to ensure that instruments are suitable for sacred use, that they are fitting for the dignity of the Church and can accompany the singing of the faithful and serve to edify them.
In fact, I’ll say that ensembles might even demonstrate a higher example: as a symbol of the Church that utilizes communities of people and their varied gifts to spread the Gospel.