Opera omnia, iussu impensaque Leonis XIII (vol. 9): Summa IIᵃ-IIᵃᵉ q. 57-122

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Why do some people hate Gregorian chant (and sacred music in general)? Probably because of sin and ignorance, the "mystery of iniquity" (2 Thes. 2:7). St. Thomas Aquinas says that sacred music is to foster devotion, so (Summa Theologica II-II q. 91 a. 2 "Whether God should be praised with song?" co.)

that the souls of the faint-hearted (infirmorum) may be the more incited to devotion.

But those who are spiritually dead are worse than "infirm"; they don't have any living seeds of devotion to foster and make grow.

Cdl. Cajetan discusses this article on DjVu pp. 328-9; he claims there were no organs in St. Thomas's era, ∵ they were not used coram the Supreme Pontiff:
Interestingly, Cdl. Cajetan's early 16th cen. commentary on that Summa article says:

quod tempore divi Thomae Ecclesia non utebatur organis. In cuius signum, adhuc Romana Ecclesia coram Summo Pontifice non utitur.
that in St. Thomas's time the Church did not use organs. A sign of this is that the Roman Church still does not use it in the presence of the Supreme Pontiff.

By organs he clearly means pipe organs (not musical instruments in general) because he goes on discussing

an pulsare in organis inter officia ecclesiastica sonos sæcularium vanitatura sit peccatum mortale.
whether playing vain, worldly melodies on the organ during the divine office is a mortal sin.

a different opinion:
"Organ, Liturgical Use of" New Catholic Encyclopedia :

St. Jerome (400) mentions an organ in Jerusalem so loud that it could be heard nearly a mile away at the Mt. of Olives. […]

Although it is not known exactly when the organ was first used for religious purposes, the writings of St. Julian of Toledo, a Spanish bishop, indicate that it was in common use in the churches of Spain by the year 450. We know that in the 7th century Pope St. Vitalian (666) introduced the organ in Rome in order to improve the singing of the congregation. As an aid to the introduction of Roman Rite into the churches of France, Pepin (714–768), the father of Charlemagne, ordered an organ from the Byzantine emperor Constantine Copronymus and had it installed in the church of St. Corneille at Compiègne (757). Charlemagne also received a similar instrument from the Eastern Emperor in the year 812, and a copy of the instrument at Compiègne placed at Aix-la-Chapelle c. 811 is reputed to have been the first organ in Germany. Apparently the art of making and using organs developed rapidly in Germany in the latter half of the 9th century, for in the year 880 Pope John VIII requested Anno, Bishop of Friesingen, to send him a good organ and, along with it, a competent player to instruct Romans in the art.

Although the organ has never been prescribed for use in the Roman Catholic Church by canon law, it has apparently been used in the Church consistently since the 9th century. By the 13th century the organ was certainly in general use throughout the Latin Church and thus was deeply involved in the development of the musical and liturgical tradition of the Church. Many of the important liturgical books refer to the organ frequently, and the fact that, though never specifically prescribed, it is assumed to be present and an important aid to the liturgy is seen by the frequent instructions of the Church that direct that it shall be played at specific times. The high esteem in which the Church holds the organ is perhaps best summarized in the following excerpt from Vatican Council II: "The pipe organ adds a wonderful splendor to the Church's ceremonies, and powerfully lifts up men's mind to God and to higher things."

(source)

also, p. 327 gives these cross-refs: "In Psalm. xxxiii, Ad Ephes. , cap. v, lect. vii."