The foundational principles of loud music during church are presented in Thom's article,
"Conventional wisdom tells us that more volume equals more energy. After all, people don't want to hear themselves sing, right?"
I believe it is thoroughly unreformed to have a church's music volume at a point where worshipers cannot hear others around them singing (the problem is amplified when I can't even hear myself worshiping).
First of all we start with the Bible. It's problematic that worship leaders are basing their practices on "conventional wisdom," especially when this leads us in a direction that might contradict Scripture. Throughout Scripture it seems as though God desires all worshipers make a loud sound of praise. The shouts of the Israelites brought down the walls of Jericho, seventeen of the Psalms encourage God's people to shout his praise in a loud voice and the ultimate picture of worship from Revelation 5 is of people lifting up the name of the Lord with loud singing. Worship should be loud. As loud as all my inmost being praising his holy name.
But does that necessarily mean that we can't have loudness in the instrumentation in addition to the congregational singing? This is where the Reformation comes into the picture.
The rationale of worship leaders who espouse loud music is that people don't want to sing or hear others singing. It's right there in the aforementioned quote. And so that logically leads them into dangerous territory: the worship team will sing for them. Amplified sound produced by professionals will take the place of the loud singing from all believers that God desires.
To me, this mindset is adjacent to prevailing ideas about worship and faith in the pre-Reformation church. Regular people can't handle the Bible, so the professionals will handle it for them. Regular people can't understand the deep mysteries of God, so the pros understand things for them. Regular people can't sing, so the pros sing for them. The Reformers rightly drop-kicked this bad theology, but it's returning thanks to the poor pneumatology of far too many pastors and church leaders.
At its worst, high volume from the professional worship team all but replaces congregational participation. It's happening all over the evangelical church. Instead of causing concern among worship leaders, this phenomenon only entrenches them further into their assumptions that people don't want to sing.
Loud volume from the worship team and instruments prevents people from being encouraged by the regular voices around them. That person standing next to me has cancer, but I can hear their voice singing "Blessed be the name of the Lord." That person sitting behind me was an alcoholic, but I can hear them singing "Grace that is greater than all my sin." That guy at the back of church might not carry a tune like Pavarotti, but he can encourage others (and obey God's command) by proclaiming "Victory in Jesus, my Savior, forever!" with a loud voice in the presence of the assembly.
It's possible that's happening in every church where the music is pumped up to 95 decibels, but I wouldn't know because I couldn't hear my wife, my kids, my neighbor or even myself.