The qualities of the room's acoustics combined with the timbres (individual sound characteristics) of the voices or instruments contribute to the overall SONORITY of the music.
That word SONORITY is commonly used among conductors. The conductor might ask the low strings to play with more sonority. Or he might ask the brass section to provide a more sonorous sound. What he means is he wants the entire section to blend together better, to combine their individual timbres to create one fat, luscious sound with no individual voices sticking out. In a choir, creating sonority is particularly difficult.
The variations in timbres (vocal colors) within a choir is a matter of how many people are in the choir -- every voice is unique. Combining alto and soprano voices singing in unison often times is like trying to mix oil and water. You might have some very bright, high voices in the soprano section and some very dark, heavy voices in the alto section. Combining them together without them "tempering" their individual characteristics would not make a very useful emulsion. The singers need to think of themselves as paint pigments. When they sing together they mix and create an entirely new color. It is neither yellow (soprano) or red (alto) but a new color: orange. The soprano voices provide the light and the alto voices provide the depth and warmth in the new color.
Having both light and dark in the sound (chiaroscuro) is what brings richness and beauty to the tone of every good choir. Tempering the individual timbres is key to making that SONORITY happen. The bright voices need to sing with narrow or no vibrato. The dark voices need to lighten up and sing with a forward placement. Everyone needs to listen louder than they sing. Having both light and dark in the sound is desirable, but only so far as blend is possible. A conductor will actually want to experiment with the different individual pigments to get a broad range of colors in his choir. But most of all he will want SONORITY within his group, with no individual voice timbres sticking out. So, remember when playing in a band or orchestra or singing with a choir to think about tempering your timbre to help the SONORITY of the group as a whole. There's nothing quite as delicious as a big, fat luscious sound!