Learn Italian: Il coro canta una canzone triste.

Il coro (the choir) is a collective noun. Just like la gente (people) or la famiglia (family), Italian treats a singular group as grammatically singular, even if it contains many humans. You must use the 'he/she/it' form of the verb (canta), never the 'they' form (cantano), even if English sometimes allows plural verbs for groups.

How do we know canzone is feminine when it ends in -e?

Nouns ending in -e can be masculine or feminine, so their gender usually needs to be memorized. However, it helps to remember that most Italian words ending in -zone, -zione, or -sione are feminine. The feminine article una right before it is your solid clue in this sentence!

Why doesn't the adjective triste end in -a to match the feminine noun una canzone?

Triste is part of a category of adjectives that end in -e in the singular for both masculine and feminine nouns. It doesn't change to match the gender, only the number. If the choir were singing plural sad songs, it would change to -i: canzoni tristi.