Learn Spanish (Spain): El coro cantaba muy bien. - The choir used to sing very well.

Cantaba is in the imperfect tense, which we use to talk about repeated or habitual actions in the past. Translating it as "used to sing" shows that the choir singing well was a regular occurrence, not just a one-time event that would use the preterite cantó.

Since a choir is made up of many people, why doesn't the verb use the plural form cantaban?

In Spanish, collective nouns like coro (choir), familia (family), or gente (people) are treated as singular entities. Because coro is grammatically singular, it takes the singular verb cantaba.

Could I use bueno instead of bien here?

No, because bien (well) is an adverb describing how the choir sang. Bueno (good) is an adjective, which you would use to describe the choir itself, like in el coro era muy bueno (the choir was very good).