Sometimes the music looks too difficult to attempt a cappella, other times it may be a song that I hate. If I don’t sing the hymns selected, have I failed to participate fully in the Mass?
It depends on why you are not singing. Are you doing it to prove some point, thus taking your focus off of God and the mass? If so, you are probably not participating fully. If you are not singing because of the difficulty of the song or because you don’t feel like singing, yet have still placed all of your heart and soul into the mass and into focusing on God, then I personally believe that you are fully participating.
I love to sing… I mean REALLY LOVE to sing. It’s part of every fiber of my being. It fulfills me spiritually. I love it so much that I make a living from it, albeit a rather modest one.

That said, even I will sometimes need to not sing when I’m at mass. Sometimes I need to just meditate on the words and music, allowing it to sink into my soul. I know that I am still fully participating even when I’m not singing. I’m singing with them in my soul. Although, I don’t get to do that often because I’m cantoring most Sundays.
Now, it’s true that some parishes do really awful music and it can be difficult to sing to either because of the music itself or poor musicianship or cantors leading the hymn. If I feel like I can get a poorly trained cantor’s voice out of my head while I’m singing, it’s not too difficult to just get past it. BUT you need strong singers in the congregation to do that, and not all congregations have that. Since I read music, I will sing most hymns. It’s also because I believe that if I can make myself sing the hymns I don’t like while I’m cantoring, then I can certainly do it when in the pews. That said, there have been a couple hymns where I just couldn’t sing they were so awful. I felt like I was mocking God while singing, so I had to stop.
The predicament I often find myself in while in the pews is figuring out how much I should sing out. As a classically trained singer, my voice is such that it almost always stands out in the congregation. When the congregation sings robustly and fully, I don’t have a problem singing with full voice. But when it is congregation whose singing is weak or near to nothing, I don’t want to sing with full voice because I don’t want people to think I’m showing off or to just bring any unwanted attention on myself. I then always end up singing in a whisper voice and that just kills me because I feel like it is only a part of my soul singing. I would rather not be singing at all when I have to be that way. Every once in a while, I can feel a congregation out and when it seems that the congregation would benefit from a stronger voice in the pews, I will sing out more and then that prompts others around me to sing out and thus I don’t become a solo voice in the pews. (I’ve actually had people thank me for doing that so that the rest of the people felt brave enough to sing.) But then other times, even when I’m in a congregation that sings out really well with the organ full-blast, I’ll have people sitting in front of me turn to me after mass to compliment my voice. As humbling and touching as that is, it is also very embarrassing because people had noticed my voice which is not what I wanted, while in the pews. When you’re cantoring it can’t be helped and I know that people appreciate someone up there who can read music, do it right and lead the congregation, as well as sing the psalms and other music well to add to the prayer. So that is the conundrum I often find myself in when in the congregation… to sing out or not to sing out, that is the question.

If I don’t sing out, is it causing me to not fully participate due to just singing half-heartedly? But, if I do sing out does it make me stand out too much or cause people to think I’m showing off? Is it sometimes better to just not sing, as much as that would kill me?