Learn French: D'où viennent ces frères sourds-muets ? - Where do these deaf-mute brothers come from?

This is a compound adjective made up of two distinct adjectives (sourd meaning "deaf" and muet meaning "mute"). The rule is that when a compound is formed by Adjective + Adjective, both parts must agree in gender and number with the noun they describe. Since frères is masculine plural, both words get an -s.

How would this sentence change if we were talking about sisters (sœurs) instead of brothers?

Both parts of the compound adjective would need to agree in the feminine plural. The sentence would become: D'où viennent ces sœurs sourdes-muettes ? Notice that the feminine of muet requires doubling the consonant to become muette before adding the plural -s.

Why is the verb viennent placed before the noun ces frères?

This is a structure known as "stylistic inversion." After short question words like où, d'où, quand, and comment, if the subject is a noun rather than a pronoun, you can simply put the verb before the noun. You could also use the heavier complex inversion (D'où ces frères viennent-ils ?), but stylistic inversion flows much more naturally here.

Are the final consonants in sourds-muets pronounced?

No, the -ds at the end of sourds and the -ts at the end of muets are completely silent. You only pronounce the vowel sounds at the end of each word.