Fr Tim has lots of interesting things to say on Music in both forms of the Mass.
My worst musical experiences have all taken place when 'doing the music' at Mass in the Ordinary Form.
(Rant alert, move on now....)
Firstly, you have to choose the music, which given how the Readings etc are all laid down is most strange and frankly, I am not hardwired to do. Some hymns are written by Protestants. Some of the music contains gross errors. I find myself trying to imagine what the Priest and Congregation will want to sing, which is a recipe for a headache. If I wonder what should actually happen, I often think no music would be best as it's usually just a distraction.
Having cast yourself adrift thus, there is then no musical gold standard to aim for. Contrast this with being an orchestral horn player where I sit in a preordained seat, know according to which part I am playing who I tune to and who I follow in matters of ensemble - dynamics, articulation, tone colour, phrasing, style etc. Unless I am the principal player, I do not speak, all questions to the Conductor going via Horn I. How rigid? No, just the most efficient way of getting up to 110 musicians playing with the greatest degree of unity.
Meanwhile in the Ordinary Form of the Mass, what does constitute musical success? The Mass Ordinary seems to take a back seat to hymns that are not actually the Proper texts of the day. The Gloria is turned into a verse chorus form even though we know that text repetition is used with great care in the Mass and the Creed usually not set because it is too long. Maybe that's why the plainsong settings are usually on the syllabic rather than the melsimatic end of the text setting spectrum.
Instruments or not? I have been made to play the piano at school Masses in a Church because the 'children/young people don't like the organ.' The trouble with electric pianos that don't have additional amplification is that you can't hear them and the bass is very wimpy. Then you usually have to sit in some place where you are in the way. I'm very happy to take a back seat, but if you are leading musically, you have to be able to do it musically.
Then you get out your singometer and notice that often many people are not singing and it doesn't matter where they are, what's being sung....they are not singing. They do not want to sing. So having dummed the music down so that we can all sing in unison (plainsong , yes I know, I'm taking about metrical Protestant style hymns here), we are not in fact all singing. Rather than blame musicians, as usually happens because I am playing too fast/slow, too many/few verses, too loud/soft, too traditional/too modern or any other criticism, I've heard most- organ/piano/guitars/ unaccompanied, forgot that one, maybe we should be bothered about whether people are able to pray at Mass rather than being worried about whether everyone needs to sing. Sometimes I wonder whether all the criticism landed on me is really because people know it shouldn't really be like this, only it has not been made at all clear what things should be like. Sometimes the congregation gets a talking to along the lines of,' it will be so much better for you if we all sing.' Maybe it will be so much better for them if they are left in peace to say their prayers. I used to sing Communion Hymns quite loudly in a support the organist way, now I often don't sing them at all, because if you are the poor soul sitting in front of me whilst I sing con belto, I am probably preventing you concentrating. 'Cheers Leutgeb, I get to go to Mass once a week and when I want to pray for my sick relatives etc, I have you intruding on my prayers....'
Thus, having cast ourselves adrift into our strange sea of musical relativism, there is nothing to aim for except 'my favourite hymn.' As a musical metaphor for conversion of heart, it's not very potent.
Having ranted on, I should say that things do seem to be on the turn. Hurray!
The Extraordinary Form lays down what you sing where. Ideally you sing the Propers properly, if not you sing them to psalm tones, but at lest you know exactly what you have to do and whether you are doing it right. All you have to do is practise.
2009, the year of Mass Propers, Plainsong and silence.
Musicians doing their thing in folk groups, choirs, orchestras whatever you like but not necessarily inflicting it on people at Mass. It's just too precious for us all to have to do a 'turn' then.