[Romulus]

I think Pius X would have frowned on Schubert's "Ave Maria" (remember: we're talking about a specific musical setting, not the underlying text) because it's modern music (not chant), in a popular style (tending to the operatic), and intended for a soloist (Pius does not forbid solos, but as I read him he does forbid soloists performing an entire piece on their own).

for a Mass on a feast of Our Lady, for example, I happen to think it would be wholly appropriate.

You are thinking programmatically, not liturgically. The textual content of the song is what we're discussing here; it's the song as a musical composition that's being considered. I'm sure there are Gregorian chant versions of the Ave Maria; if so they would be much more faithful to Pius's regulations.

Is Ave Maria not conducive to prayer?

By which you mean the Schubert composition? Of course it's conducive to prayer -- private prayer. It's a pious song, and a very beautiful one. And I don't deny that it's conducive to prayer in church as well. But other musical forms can do that as well, while not contradicting other important principles.

I'm just trying to figure out why "Salve Regina", for instance, would be appropriate while Ave Maria wouldn't.

Whose "Salve Regina"? Again: this isn't about the text; it's about the music. A chanted Salve Regina is fine; an operatic or rock or rap one would not be fine.And I'm wondering why hymns/songs must be able to be sung by the congregation if they are to be used.

Because liturgy is not a time for private devotion. At Mass, we all have our proper parts, but popes have been encouraging the laity to participate actively (at the appropriate times) since long before Vatican II.

I know you're not argiung. I appreciate your patience, and hope you don't think I'm being perverse. It's just that as sacramental people, everything we do has symbolic importance. Using signs is bound up with how the church prays, taking minute care that what we imply in art, music, word, and movement are integrated and coherent.

[ninenot]

Because liturgy is not a time for private devotion. At Mass, we all have our proper parts, but popes have been encouraging the laity to participate actively (at the appropriate times) since long before Vatican II.

Maybe, maybe not.

The term 'actuosa participatio' REALLY means "actual" or "real" participation--and as has been argued by highly-credentialed liturgists, this begins internally, with a 'metanoia' or conversion, of oneself to a conformity with Christ.

This metanoia does not necessarily require activity, song, or verbal response.

Thus, art song which is religious and not merely a vehicle for "performance" is perfectly acceptable.

Again, context counts. One solo, done without excessive melodrama or vocal floridity, is not a violation of liturgical norms, IMHO.