There's two things we're looking at here.
1. Are the congregation singing the word?
2. Are the congregation singing heartily?
To assess Q1, you need to look at the term's song selection. Do the songs deeply reflect the concerns and passions of God's Word?
To assess Q2, you need to look at how well the congregation are singing. Are 30% of the people just standing there waiting for the song to be over? Are people engaged with the singing? Are they getting into it? What sound does the visitor to church notice? Is it the sound of the people out the front or the sound of the congregation? I want people to be blown away by the sound of the congregation.
But people often don't sing heartily (or at all).
Here are some reasons I've thought of and some solutions.
1. The congregation are embarrassed.
The music team have chosen songs that are embarrassingly dated or daggy or maybe the musicians are poorly rehearsed or just incompetent. Whatever the reason, the congregation feel embarrassed about the music and so they don't sing.
Solution - Don't choose embarrassing songs. Rehearse properly.
2. The congregation is nervous.
The girl leading the singing is visibly uncomfortable and her fears are transferred onto everyone else. The whole congregation feels her nerves and so doesn't sing well.
Solution - Rehearse more so the song leader won't be so nervous. Have her lead with someone more experienced.
3. The congregation is overwhelmed.
There is a tidal wave of sound hitting the congregation from the front. All their energies are spent processing that sound. Who has any energy left to sing? And what difference would it make if you did sing?
Solution - Turn the volume down!!!
4. The congregation is unconverted.
Why sing songs about Jesus when you don't care about him?
Solution - Preach the word.
5. The congregation is unfamiliar with the song.
It's new and strange and I don't know it. How can I sing it heartily?
Solution - Sing fewer songs more often and teach songs better.
6. The congregation feel exposed.
There are so few people / such poor acoustics / such poor leading that individuals don't want to sing because they think everyone will hear them
Solution - sit people closer together, rent-a-crowd, lead better.
Can you think of any more?