The Divine Command to sing, proper antiphons, and you.

[pnewton]

You are wrong.
It is a sin of grave matter not to participate (which means attend) in Mass.
It is NOT a sin of grave matter not to sing at Mass.
Big difference.

You keep changing what has been said. No one has said “grave matter” but you. Anytime we deliberately choose to do something that God has told us to do it is a sin. Since the matter isn’t grave, it is a venial sin. However, God’s command on this is general. This means there may be many reasons one does not choose to participate in this part or that part of the Mass. My opinion, making a choice to refuse to sing with zero reason is a violation of what God has asked us to do, and is thus, a sin. You have a different opinion maybe, but that doesn’t make anyone else’s opinion “wrong”.