1. Perhaps it is that when they sing a hymn or a song they pray with words as they sing aloud and and they pray with the heart as they hear the words they have just sung.
2. Perhaps it is that the words they sing are more memorable than the words they speak, so they tend to sing them to themselves afterwards and thus repeat the prayer.
Perhaps it means something else - but its haunting and elusive profundity reminds them that music, and singing in particular, has been a central feature of much Christian worship.