Music has always been a Big Thing. The early Christians believed that only vocal music was suitable for Mass, and you will find a fair number of Christian sects in the US who still feel this way. The Catholic Church has gradually adopted the use of instrumental accompaniment and various styles of music, but the rules are still pretty picky. Unaccompanied singing, preferably of chant music, is still our liturgical default. (There’s a reason it’s called “a capella” – “at the chapel.”)
MIDI is a programming language. If I have a keyboard, and I program it in MIDI to play a song when I hit a button, it is still being played live. Not the best way to do things, but it’s within the letter of the law in some interpretations. Other people would compare it to a mechanical music box or player piano, which constitute instruments “operated mechanically or automatically.” Such instruments are forbidden at Mass by the 1958 “De Musica Sacra,” which demands that any instrument played at Mass be “played by the personal action of the artist.”
Article 32 of the Directory for Children’s Masses is the only official instruction that has ever permitted use of recorded music at any kind of Catholic Mass. It reads as follows:
'"Care should always be taken, however, that the musical accompaniment does not overpower the singing or become a distraction rather than a help to the children. Music should correspond to the purpose intended for the different periods at which it is played during the Mass.
“With these precautions and with due and special discretion, recorded music may also be used in Masses with children, in accord with norms established by the conferences of bishops.”
About three minutes later, several bishops’ conferences banned use of pre-recorded music even at kids’ masses… so you can see how that went over.
For those of us in the US, recorded music is a no-no unless it’s a kiddie Mass, even under the most recent regs, “Sing to the Lord.”
"93. Recorded music lacks the authenticity provided by a living liturgical assembly gathered for the Sacred Liturgy. While recorded music might be used advantageously outside the Liturgy as an aid in the teaching of new music, it should not, as a general norm, be used within the Liturgy.
"94. Some exceptions to this principle should be noted. Recorded music may be used to accompany the community’s song during a procession outside and, when used carefully, in Masses with children…
“Recorded music should never become a substitute for the community’s singing.”
That said, if you go to Mass and your priest uses one of the Eucharistic prayers for kiddie Masses, he has a right to use any of the other kiddie Mass permissions that he feels like, and he may already be using them.