Answer:
The part of the song that most emphasizes its singers’ demand for freedom is the repetition of the line “Let my people go”.
Step-by-step explanation:
The question is not complete since it does not provide the reference lyrics to answer it, here are the lyrics:
Read the song lyrics. 
 Go Down, Moses 
 "Go Down Moses" is a negro spiritual that references the Old Testament and the events of Exodus. 
 
Go down, Moses, 
 Way down in Egypt land 
 Tell old Pharaoh 
 To let my people go. 
 
When Israel was in Egypt land 
 Let my people go 
 Oppressed so hard they could not stand 
 Let my people go. 
 
 Go down, Moses, 
 Way down in Egypt land 
 Tell old Pharaoh 
 “Let my people go.” 
 
“Thus saith the Lord,” bold Moses said, 
 “Let my people go; 
 If not I’ll smite your first-born dead, 
 Let my people go.” 
 
Go down, Moses, 
 Way down in Egypt land, 
 Tell old Pharaoh 
 “Let my people go!” 
The repetition of the line “Let my people go!” convey the message of urgency for freedom in many ways, first of all by being present every four lines it makes it a clear and strong message, besides the last time that it is said, it has an exclamation mark which gives it a bigger sense of urgency and importance.