On Some Musical Differences between the Extraordinary and Ordinary Forms of the Massposted by Michael E. Lawrence
In the course of the past few weeks, I have seen evidence that there is some confusion regarding music as it applies to the Extraordinary Form of the Roman Rite. Some are applying the Novus Ordo rules to the Traditional Mass. In light of this, I thought I'd publish a short clarification on a few points.
The following points presuppose a High Mass; Low Masses are a bit different, and kind of confusing in some respects. Perhaps I'll post on the Low Mass at a later time.
1. In a sung Mass all of the Propers must be sung. They do not necessarily need to be sung according to the authentic chants in the Graduale Romanum, although it is surely a pity when they are not, but they need to be sung. One could sing the Propers to a Psalm tone instead, as well as recto tono; though musically unsatisfying, this does fulfill the obligation to sing them.  (Also, one colleague of mine with encyclopedic knowledge and whose judgment I trust instinctively has written to say that one could sing a falso bordone setting of the Proper as well. "The more modal and Renaissance versions are particularly fitting," he writes.
There are also many good polyphonic settings of the Propers. Perhaps the most notable is the Gradualia by William Byrd. Visit cpdl to find such things.  Of course, new polyphonic settings can be composed as well. [2a]
2. The Graduale Simplex does not apply to the Extraordinary Form, and it cannot be used with it. It is not an option. 
3. Hymns cannot replace the singing of the Proper antiphon. This is a crucial point, and one which many may be quite unaware of, however obvious it would seem to most of the readers here. A Latin Hymn (e.g. Adoro te devote) could be added, for instance, at Communion, once the Proper has been completed. 
4. Nothing in the vernacular can be sung during the Mass. Even extra music besides the Propers must be in Latin. 
5. Vernacular hymns can be used while the priest processes to the altar. But they must cease once he gets there. Vernacular hymns can also be used at the conclusion of Mass. Neither of these times are technically part of the Mass.
6. The Ordinary of the Mass must be sung in Latin in all High Masses. Pride of place is held by the very fine Gregorian chant Ordinaries which can be found on the CMAA's website and in which the congregation can join in singing. It is, however, not necessary that these be used all the time. There are some good Latin settings of the Ordinary which the congregation can sing. There is one by Healey Willan that is quite good. Don't forget the polyphonic choral Ordinaries. They are quite fine indeed.  Again, new settings can also be composed. [6a]
7. Finally, make sure you are using the right chant books. Use the 1961 Graduale Romanum which can be found again at MusicaSacra for free download (or you can buy a nice copy for a reasonable fee). The later books published by Solesmes are for the new rite, and it is in many cases completely impossible to reconcile what appears in those books with what is required by the Extraordinary Form of Mass. The upcoming Feast of the Assumption is a really good example. I remember the enthusiastic but naive young man who showed up at the Assumption Mass in Camden a few years ago with a 1974 Graduale Triplex. By the middle of the Gradual, he was lost. That's because the chant actually changes halfway through in the 1974 book.
What was that guy's name again????? ;)
In any case, if all of the above was obvious to you, that's good. If you learned something, that's even better. Whether you learned something or not, please print this out and give it to anyone who breathes a word about singing a High Mass. Let's do it right.
 Sacred Congregation of Rites, De musica sacra et sacra liturgia, 21 b and c.
 Ibid., 17 and 21a.
[2a] Mediator Dei, Encyclical Letter of the Servant of God, Pope Pius XII, 193.
 Ibid., 21c.
 Ibid., 14a. See also Musicae sacrae disciplina: AAS 48  16-17.
[6a] Mediator Dei, 193.