It seems obvious, then, since the Lord calls us to be good stewards of our physical bodies, we need to be careful with the volume levels to which we subject ourselves and others. We dare not damage our hearing and/or the hearing of others simply because we like loud. Such a scenario would clearly demonstrate a lack of good stewardship.
So, to the point of how loud is too loud for church music, I recommend investing in a sound level meter (decibel meter) for your church. You will then be able to monitor the level of sound and thereby be careful with the volume.