- Expressive of prayer
- Unanimous participation of the assembly at the designated moments, and the solemn character of the celebration.
You could summarise this as saying that it's got to be something prayerful and solemn that someone having no special musical training can actually sing.
Paragraph 1158 says that songs are
"all the more expressive and fruitful when expressed in the cultural richness of the People of God who celebrate".
So contemporary styles of music, which are a part of our cultural richness, can be used.