Most sound engineers are just volunteers who know where things get plugged in. Their egos let them believe that they are important and that their desired volume level is ideal. The musicians tend to agree with these engineers because their egos like louder volumes, too. The only people who matter, the audience, are typically dismissed as cranks if they disagree with the volume level.
Real, professional sound engineers know that the ideal volume level should seem invisible to the audience. In other words, the audience shouldn't notice that the volume is too loud or too quiet.
It is too bad that your comment was dismissed. Professional stagehands look for ways to please the audience because they know that they are out of work without the audiences. Unfortunately, even the best stagehands are paid by the musicians who usually believe that louder is better.
Walking out and asking for a refund is the best way to convince the venue, the musicians and their stagehands that they were wrong. But, in a church setting, I suspect that simply leaving wouldn't educate anyone. Speaking directly to the church leaders, especially when they ask for donations, is the best way to get your point across.
Of course, you could write to them anonymously and try to explain how many people with ASDs can't enjoy loud sounds.