Working in a group with individual mikes requires people to match as closely as possible their mouth to microphone distance. Different proximity alters the quality as well as the volume of the sound. Just because one person sings more loudly than the others, or more softly, doesn t mean that he or she should swallow the mike or move it out to arms length to compensate. It is the job of the sound operator to balance the voices. Sound checks are obviously important.
Sound operators are notoriously uncommunicative.
The volume on stage is your prerogative, not his. So hire a rehearsal studio at least once before a microphone performance. Familiarity overcomes most technical problems.