"The 144,000 people sang a new song." This explains what the sound was that John heard. AT: "That sound was a new song that they sang"
"living being" or "living thing." See how you translated "living creature" in Revelation 4:6
This refers to the twenty-four elders around the throne. See how you translated "elders" in Revelation 4:4.
"one hundred forty-four thousand." See how you translated this in Revelation 7:4. (See: rc://en/ta/man/translate/translate-numbers)
Possible meanings are 1) "have never had immoral sexual relations with a woman" or 2) "have never had sexual relations with a woman." Defiling oneself with women may be a symbol of worshiping idols.
Possible meanings are 1) "they did not have sexual relations with a woman who was not their wife" or 2) "they are virgins."
Doing what the Lamb does is spoken of as following him. AT: "they do whatever the Lamb does" or "they obey the Lamb" (See: rc://en/ta/man/translate/figs-metaphor)
"Firstfruits" here is a metaphor for the first offering to be made to God in celebration of harvest. AT: "purchased out of the midst of the rest of mankind as a special celebration of salvation" (See: rc://en/ta/man/translate/figs-metaphor).
Their "mouth" refers to what they said." AT: "They never lied when they spoke" (See: rc://en/ta/man/translate/figs-metonymy)