This is a CRUCIAL stage in sound, NEVER assume that everything will sound good just because it did last week.
Each new worship session has musicians and singers coming in with a different level of ‘warm up’ stage. Each individual has a different volume and this changes as they get more and more warmed up during rehearsal AND live worship.
Sound check allows you to get setup for this and get the best sound, it is the foundation. Get this bit wrong and you will struggle to make it right during worship. Remember, because its live you only get ONE SHOT.