When listening to a sermon together, there is a congregational shared presence — we know and are aware of each other’s presence. We are “listening” to each other listen to the sermon. We are all listening together in community, and being preached to as one flock.
Even when only one person is praying a congregational prayer, we are all present and listening to the same prayer. We are joining in with the prayer to God. We are both its audience and its co-participants, even when silent.
How so? I think part of that comes from shared presence and awareness that we are involved in the activity together.
Similarly, Sunday-morning singing is normatively (it ought to be) mutual and communal and congregational and shared. We are aware of others singing with us — through hearing them, occasionally seeing them, in addition to just being simply aware that they are there in the room with us. We are singing together.
When whatever is happening on stage or with the volume/speakers inhibits being aware of our sharing in the activity together — when we can’t tell that others are singing, or perceive that others are joining in with us, or when we reasonably doubt that others are singing, the singing becomes less communal or congregational. And we’re missing out on mutual encouragement.
“Let the word of Christ dwell in you richly, teaching and admonishing one another in all wisdom, singing psalms and hymns and spiritual songs, with thankfulness in your hearts to God.” (Colossians 3:16)
“Addressing one another in psalms and hymns and spiritual songs, singing and making melody to the Lord with your heart.” (Ephesians 5:19)
Addendum (Oct 29, 2021). This is why I am partial to congregational singing, subtle instruments, minimized amplifiers, and worship bands “backing off” to let the congregation take over.