Well, my experience with these things has included singing (in a choir, mind you) in Westminster Abbey, Westminster Cathedral, York Minster, Chiesa della Gesu, San Ignatio, the chapel of Hungarian Saints (in the crypt below St. Peter's) and in St. Peter's near the Altar of the Chair of Peter, as well as attending regular services for about 1 week at the Brompton Oratory, and attending Masses at several churches and cathedrals around the US over the course of my life.

Here's what I think works best: 1) high walls with a narrow nave, 2) side aisles with pillars and vaulting, 3) a clerestory, 4) the organ should be placed either in a tribune at the liturgical west end of the nave, as high as possible, or on a screen at the crossing, again as high as possible, 5) the choir should be ideally in the tribune with the organ, or in a gothic choir arrangement, that is, stalls facing each other, east of the crossing (with or without a screen).

Basically, you need reflective material, but you also need "treble traps" and other irregular surfaces to help make the decay of the sound be smooth (the difference between "echo" and "reverberation.") Even with hard walls, wooden ceilings (truss ceilings, etc.) pose their own problems, as the wood can be sound-absorbing. Wooden pews for some reason are different in the above-described, but that may have to do with the juxtaposition of the organ being high up and the irregularities of a vaulted ceiling as against a wooden ceiling.

Carpet your living room, not your church. Not even under the pews. No pew cushions, either.

One of the real problems I've noticed, and this is sure to start a whole different discussion, is that when the accoustic is good for music and congregational singing, especially with choir/organ, it's not so good for spoken word or orchestral accompaniment (if you're doing orchestral Masses).

The short answer is, build it like the old cathedral and parish churches in England and the Continent, not like the shopping malls and sports arenas of the U.S.

Pes:

Well, I hate to throw a wrench in the works, but what If we are playing guitars and a piano from the altar, or close to the altar? Then the acoustics described above just won't cut it.

Could it be that churches were built the same way century after century because the music of the church before the last century was complimentary to those types of architectures and vice/versa? Don't you think that only the organ and/or choral singing work in those types of environments?

If that is the case, then is it true we must surmise that the environment for the liturgy is intricately joined to the music that sounds within its walls? Could we stretch this thinking to say that to stray from the architecture of the historical church means to abandon not only it's historical architecture, but, in a sense, it's liturgical development, it's tradition, it's music and everything that has gone before? Is this perhaps why "Environment and Art in Catholic Worship" had to be crafted in the late 70's?

Here are four steps on how to remove sound-deadening (and faith demoralizing) material.

1. Rent a bulldozer
2. Bring it to nearest church building in the round
3. Turn it on and shove it to the ground
4. Build an authentic church where it once stood