Learn French: Où sont mes clés de voiture ?

The accent on the u doesn't change how the word sounds, but it changes its meaning. Où with an accent means "where", while ou without an accent means "or".

Why is it mes clés instead of ma clé?

In French, words for "my" must match whether the noun they describe is singular or plural. Because clés (keys) is plural, you must use the plural form mes. It doesn't matter if the noun is masculine or feminine when it is plural.

Why does the sentence use clés de voiture instead of clés de la voiture?

When one noun describes the type or purpose of another noun, French connects them with just de, dropping the word for "the" (la or le). So clés de voiture acts as a general category meaning "car keys". If you said clés de la voiture, it would mean "the keys to the specific car".

Do you pronounce the final letters in sont and mes?

No, the final t in sont and the final s in mes are both silent here. In French, final consonants are typically silent unless they link to a following word that starts with a vowel sound. Because mes and clés start with consonants, there is no linking sound.