"Listen to each other and don't sing louder than your neighbor." This is a common command from the choir director. Although these instructions are good and will help with overall blend of voices in the choir, there may be more going on. Sometimes a few people simply don't hear their own voices in context of the full group. Sometimes the singers don't know how to match pitches when their parts include awkward jumps and skips. Sometimes they are not aware of techniques that help adjust their vowel molds towards better tuning or help them support their tone.
Here are a few tips about what to do for these problems as a choir director of a small all-volunteer group:
1. Always do a few vocal warm-ups at the beginning of practice. Choose a warm-up that targets something particular to guide the singer's listening. This will allow you to guide them in practicing good breath support, tone inception, dynamics, vowel mold formation, etc.
2. Isolate any problematic issues in the song. Point out what may cause the problem and practice solutions. For example, the line might include a sudden skip of an awkward interval. The issue may be solved just with better breath support. However, the real issue may be that the initial sound is on an unvoiced consonant like an "s" or a "swallowed" consonant such as an "l." In that case, the singer must use good listening as well as breath support to quickly sing through the problematic consonant to land solidly on the vowel.
3. If the phrase begins on a vowel, have the choir practice "breathing in the shape of the vowel." This will help tuning from the inception of the tone.
4. Have a "ringer" in each section. If at all possible, recruit at least one good singer to anchor each of the vocal sections. For those singers who cannot hear themselves sing or lack confidence, they tend to match pitches better when the vibrations of the other singers help them.
I recently visited at a ward choir practice where there was one youth in the bass section who sang the part an octave lower. Even though the choir director tried to encourage all of the basses to sing in the right octave, he still continued his basso profundo. About halfway through rehearsal, another bass showed up. He clearly was able to read the music well and had a good voice. He sat right next to the kid who was dropping the octave. Suddenly, magic happened. The kid started singing in the right octave! It really pays to have "ringers" in each section to anchor the part!
"Sing Alleluia" 3-Part Round is good to use as a choir warm-up. Great for tuning thirds, practicing shaping the vowels and rhythmic timing. It also addresses some problems: beginning on an unvoiced "s" and a swallowed "l" as well as jumping up and down suddenly on a large interval, a "5th."