There are, of course, nonsense syllables in many languages, and some songs also have archaic/poetic vocabulary that could mislead the language student. Even in church music you get long choruses on variants of words like "Alleluia" and "Amen" which don't carry much more meaning than "tooraly-ooraly-addity".

But overall, traditional songs are probably more likely than "art" songs to be phrased according to the natural stresses and cadences of speech. The "folk process" tends to tweak the song so that the words are easy on the tongue and on the ear. Many hymns and carols, too, are mainly one-syllable-per note, at least in cultures where the meaning of the text was considered the important thing.

Opera singers do stretch and distort the vowels in order to maximise volume and the beauty of the sound, which sometimes seem to take precedence over meaning and communication. But I would hope most of them are fairly conversant with the languages they most often sing in, particularly Italian.

Marje