Be reasonable - try to work things out. If they are hired employees, it's one thing, but most people in Worship Ministry serve as volunteers. Listen to the needs of your musicians. Reach a compromise if you need to - after all, it's a dress code, not Bible.
This issue may be just a symptom of a deeper problem - there may be a power struggle between MOM and musicians going on. Please, ask yourself - is this about keeping a good appearance on the platform, or about enforcing one's agenda to control others?