Invest in training and supervision . You don't have to accept poor workmanship from volunteers, even though it's offered without pay. That means if someone is tone deaf or repeatedly sings off key, you shouldn't feel obligated to let that person torture the congregation. The person should be encouraged to develop gifts other than singing. Likewise, if a person freezes in front of a group or can't speak clearly, that person probably shouldn't be asked to teach a Sunday school class or lead an adult Bible study.
Some firemen are volunteers.
Candy stripers in hospitals are volunteers. Many teacher aides are volunteers. All of those volunteers work within a system, following the rules and responsibilities of their professions. They go through training, commit to service, and submit to supervision, evaluation, and correction. Likewise, the church should have a system for training and supervising volunteers.