"I have never sung this."
Translation:Ik heb dit nog nooit gezongen.
It is not strictly necessary, but in most cases you want it there. Let me try to explain. "Nog" in this context means "yet", but it does not hold such a strong promise for the future as it does in English. In a way it puts the emphasis of the sentence on the fact that it has not happened yet, but it might. Leaving out the "nog" means that the emphasis must fall somewhere else. For example:
"Ik heb dit nooit gezongen!" - "I have never sung this. (It was that other guy!)"
"Ik heb dit nooit gezongen." - "Never have I sung this."
"Ik heb dit nooit gezongen." - "I've never sung this. (It was a spoken word performance!)"
You get the gist! It is subtle, but "nog" does play an important role here.