Another day, another problem. My choir is supposed to sing at a mass by our bishop. I don't intend to go.

I would appreciate it if anyone could provide some sources which state attending non-Catholic worship services is prohibited.

It seems to me that this is an ongoing issue for you.  You should probably remove yourself from the choir.

Passive attendance can be permitted for a relatively-serious reason, provided there's no scandal, but singing during a Mass is hardly passive.

I agree that you should simply leave this choir.

So... Here is the issue.  Anyone besides a traditional Catholic probably won't take the argument of your saying that a Novus Ordo service is non-Catholic worship because it is approved by "the Pope".  I would just be simple and tell the person in charge that your religious beliefs do not allow you to sing at a Novus ordo service.  Then if they ask you further you can say more. 

I'll definitely tell him I can't go. I was just wondering if I can say something better than it violates the first commandment.

Not saying anyone in your choir will be open to it, but you never know what God has in mind. 

I got one Novus Ordite to start looking into sedevacantism already, I don't want to leave the choir because we sing nice Catholic hymns and because there are many sincere Novus Ordites there. Obviously I will leave the choir if Maestro won't allow me to skip Novus Ordo services, until then it's no problem. The funny thing is we have a Protestant in our choir and he has no problem singing Mass and hymns to Mary and the Eucharist but I have a problem with the NO

Keep fighting the good fight! 

Will do.

Another day, another problem. My choir is supposed to sing at a mass by our bishop. I don't intend to go.

I would appreciate it if anyone could provide some sources which state attending non-Catholic worship services is prohibited.

I’m confused, why would you have a scruple about singing at a mass celebrated by your bishop?

Well, if your goal is to convert them to Tradition, and the choir director is a Prot, you could say something along the lines of their having changed the Mass so that it now resembles a Prot service and that Catholic are not permitted to participate in Protestant services.  That might trigger some thoughts and questions on his part that could lead to additional conversation, such as:  "I thought the Catholic Church says it's OK to attend Protestant services?"

The choir director is a Novus Ordite of the freemasonic variety, although he is moderately traditional with his repertoire.

There are so many good things you can learn from being in a choir that is at a professional/academic level, as long as the repertoire does not contain sinful music (honouring false gods/heretical "Christian" music/sɛҳuąƖ pop songs etc). Musical knowledge can be used in true Catholic worship, and other things (teamwork and communication skills, humility etc) will help you everywhere.

My advise is, every time the choir puts you in such situations (having a performance at the Novus Ordo, having to participate in a competition on Sunday etc), be honest to the director/president about your position. If these only happen once in a while, and only make up a small part of the choir's activities, it's not necessary to leave.

In my experience, directors/leaders usually respect choristers who voiced up up front about not feeling comfortable singing certain pieces or in a certain context. If they don't respect your beliefs, you walk away. Do be polite, and do your best outside those situations (know your parts well, befriend people, be a good member) so that they can see you are not trying to be difficult. And inform them as soon as you can, don't do it at the last minute.

If however, those are a big part of the choir's activities such that your non-participation becomes a problem to them, normally the director/leaders themselves will ask you to leave.

Just for context, I managed the choir (professional level) in my university.

Will you be singing "On Eagle's Wings"?
hahahaha

Our repertoire will probably be acceptable even at a TLM. As I said, our maestro is traditional in his choice of music.

Also, we're not Americans.