As stated by others and from my own personal experience as a singer, I don't sing if the volume is so loud I can't hear myself. I don't want to thrash my voice.
I recommend 80 to 90 dB (depending on the makeup of the congregation) for worship where congregational singing is desired.
That seems to be a good volume for people to join in and be able to hear themselves sing.
When it comes to a concert or presentaton (choir/soloist) 90 to 100dB, though I've enjoyed higher. I personally take earplugs to concerts to protect my ears. Maybe not important for a person who attends concerts on occasion, but since I am exposed to high volumes on a regular basis, I take extra percautions to protect my hearing.
There are professional audio techs who mix with earplugs (expensive, molded, flat response) after the first couple of songs. They are the first to arrive and last to leave and therefore have the longest exposure to high (potentially damaging) volumes.
I don't sing if the volume isn't loud enough to cover up my terrible singing voice! People like me who cannot carry a tune if you put a handle on it just don't want to be heard. After all, the Bible says to make a joyful noise, not noise that'll have those standing around you covering their ears. ;-)
I recommend 80 to 90 dB (depending on the makeup of the congregation) for worship where congregational singing is desired.
I'm not a professional, been working the sound boards for longer than I care to remember though and 80-90db sounds about right - depending on the size of the room. Our church is big enough that 80-90db coming out of the mains in the front of the church is about 40-50'ish db in the back. Been saying we need more speakers towards the mid and rear of the church to level out the sound. Maybe someday...