sn The instruction to “open your mouth” is a metonymy of cause; it means “speak up for” (so NIV, TEV, NLT) or in this context “serve as an advocate in judgment” (cf. CEV “you must defend”).

sn The instruction compares people who cannot defend themselves in court with those who are physically unable to speak (this is a figure of speech known as hypocatastasis, an implied comparison). The former can physically speak; but because they are the poor, the uneducated, the oppressed, they are unable to conduct a legal defense. They may as well be speechless.

tn Or “of all the defenseless.” The noun חֲלוֹף (khalof) means “passing away; vanishing” (properly an infinitive); in this construction “the sons of the passing away” means people who by nature are transitory, people who are dying—mortals. But in this context it would indicate people who are “defenseless” as opposed to those who are healthy and powerful.