Sometimes people don't want to sing out in church. They mumble their way through the congregational responses -- whether spoken or sung. They sing with a subdued voice during the hymns and canticles. When nobody else is singing (or the acoustics are so bad that it seems that no one else is singing) it is very discouraging for people, and we often have the tendency to pull back so we don't stick out. But then it just cycles worse and worse: there are fewer people singing because they hear fewer people singing, which makes the singing even less, etc.
My pastor has told us many a time how important it is that we sing out in church, and that we speak the responses full-voiced. There are people in church with us who are grieving the loss of loved ones, and people who are enduring satanic attacks, and people who are depressed, and people who are struggling with family problems, and they may be unable to sing because of the depth of emotion goin' on there. It is our ministry to our needy brothers to sing for them. They need to hear the congregation singing when they can't. They need people praying those words for them when they are too weak and too beaten down. And you experience that for yourself when you are trying to sing, but the tears choke your own voice, and you need your neighbor singing those words when you cannot.
"Speaking to one another in psalms, hymns, and spiritual songs" is most concretely done in the Divine Service. "The mutual conversation and consolation of the brethren" is something that is done obviously and frequently as we speak and sing in the Divine Service.
I regularly attend two congregations that have very very different personalities, so I've observed something. Forget for just a moment that the main reason we each go to church (and participate) is because of what God does for me there, what He does to save me and love me. There's also something else that I have become quite aware of in the last several years. I am greatly encouraged in the Faith when I am at church with others who yearn to be there, others who sing and participate, others who desire to soak up the Word of Forgiveness. It is discouraging to our neighbor and makes light of the importance of his faith when we choose not to participate in worship. When we attend church weekly, and go to Bible class, and sing in church loud enough for our neighbor to hear, that is one of the greatest ministries we are privileged to engage in.
Now, I don't think pointing this out to a mumbler is going to encourage him/her to sing out. It would probably be a law-motivation to do so, anyhow. ("Hey, buster, you'd better sing so that you can do your duty for your neighbor!") We participate in church because what's being said and sung is our life and strength, without which we're done for. But if people believe that the liturgy and the hymns are their very sustenance and life, annnd if it's also pointed out to them how much it means to the little old lady in front of us to hear our voices, who knows, maybe the love in them for their neighbor might move them to "preach" God's word (by singing out the hymns and liturgy full-voice) to their fellow worshipers.