A shallow vaulted ceiling is better than a flat ceiling. Most churches build their vaults running front to back so that there is a high wall section at the front. However, a shallow vaulted ceiling actually sounds better if the vault runs side to side. This is because the sound is stored in the high volume part of the space, which is located under the peak of the vault. So, the sound is stored side to side in the area where the people sit.
Furthermore, people’s ears are separated sideways so we are more sensitive to side to side sounds. People tend to like the spacious effect of side to side sound storage. A slightly sloped ceiling at the front of the church moves sound away from the front towards the congregation, keeping the launched sound clear and unmuddled. A slightly sloped ceiling towards the back of the seating area compresses the sound coming from the front making it louder for people sitting further away, which is good, too.
A slightly sloped ceiling still keeps the sound of congregational singing within the congregation, where it belongs, just like a flat ceiling does, which is good. Flat ceilings are not good, though. All sounds that hit the flat ceiling are reflected back down to the floor at the same time. The timing for floor-ceiling reflections is the same everywhere in the church. This creates a horrible droning tone problem.
Remember that, no matter whether you build your church with flat ceiling or sloped ceiling, or which direction you aim your vault, you will still need to address the acoustics of the space. This means you will typically need ASC acoustical treatment on the back wall and on the side walls.
The front of the church is usually built with a raised stage that is carpeted. You’ll want to make sure the stage is quieted from drumming and thudding sounds by building it with ASC WallDamp between the joints and filling the cavity under the stage with ASC SonicSnow. The front of most churches can be left fairly reflective.
Choir areas need proper design. Don’t put them out in the open, they need reflections returned to them so they can hear themselves and stay in tune. Don’t use carpet under the piano or choir. Carpet the speaking area and, if there is a praise band, you’ll need additional ASC acoustics to get the loudness under control and to get the mics working right.
Use one high central speaker cluster for speech and two side main speakers for music. Only the soloist should sing through the central cluster.
Things always work best if you design the church from the beginning around the acoustics. After all, if people can’t hear what’s being said, they stop coming.