The new testament doesn't authorize instruments. They used instruments in the old testament after Christ nailed it to the cross that ended. We are not subjected to the old law only for learning we are under the new testament law.
The Restoration Movement from which churches of Christ were born sought to emulate the earliest Christian churches where instrumental worship was not practiced. The translation of "a capella" is "like the church". The beauty and strength of churches of Christ is their unity in Christ but independence from each other. Each congregation's elders prayerfully rule on the issue of instrumental worship and many other issues that could divide His flock. Members must respect their elder's decisions and "With all humility and gentleness, with patience, bearing with one another in love.” (Ephesians 4:12)
Thanks! Your answer is awaiting moderation.