If someone sings a bad solo in church and then asks you how they did, do you say, "It was bad"?
I am a musician, a trained singer and flautist. I have played professionally and taught music. I know music well.
So this comment is a really hard one for me. Both for me and others. Last Christmas, I accepted a request to lead the Christmas Eve service. I forgot about my dislocated jaw. Its ok, unless there are a lot of high notes. Well the pianist told me he wouldn't go above D in the middle of the staff, which I can handle. He must have forgotten about one carol. It was in the high F range. And not once or twice, but what seemed like a hundred times in those 10 verses (Of course, the only song that had that many verses!)
About verse 3, I could feel my jaw locking, but I pushed it back into place. By verse 5, I was in serious trouble. Stupid me! My jaw got stuck open on verse 6. I had to just work on the jaw, and let the congregation sing it on their own.
So what was the reaction afterwards? I had so many people coming up and telling me what a beautiful voice I had. Ok, my voice isn't bad, and I can sing on pitch. But I considered it a disaster, even if it wasn't "all about me."
Some people said nothing. That is my reaction to someone who sings terribly. Sneak out and don't say a word. No one even pointed out the major failure I had. But then again, I think it was pretty obvious that most people knew about my RA and figured this was just another RA issue. (It was - my jaw dislocated from an RA flare in Feb. 2015, and it has never been quite the same since.)
And were all those people who told me I had a lovely voice lying? I don't think so. When I played in bars, I used to lose my voice every week. It took me till I was saved 10 years to learn I had bad allergies to cigarette smoke, although in those days, it just made my vocal chords swell up. Nothing permanent, no ER.
In the bars, I would sing terribly, and people, drunk people would always tell me how much they loved my voice. They went out of their way to tell me. Were they lying? I don't think so - I think that either they were clueless about music, or they were too drunk to figure it out. So that gave me a grain of salt, when dealing with church people.
I do think that many people do not really know what good music sounds like - including those who decide they want to be singers and are tone deaf and do not even have the throat to be able to produce good vocal sounds. So would I tell a bad singer they need to improve? Not likely! It is hard to train and older person to sing on pitch, or have good quality vocal production and sustain notes. I taught elementary music for 7 years, and my focus was always on teaching monotones to sing, teaching those with mediocre voices to sing well and those with good voices to sing better. And the occasional opera star? Well, I told their mothers to put them in singing and piano lessons. Not everyone did, of course. And I did produce some good singers, I had music teachers who took over my school tell me how good the children sang!
So back to the OP - I would never lie, not even to save someone's feelings. But I would avoid the issue if possible. Maybe that is a sin of omission? We do need to care for the feelings of others, so it is a fine line to walk. But lying, is wrong, except in those impossible situations like hiding Jews from the Nazis, or people who smuggle in Bibles to closed countries. Those are lies to counter evil, and I guess that is between the people and God.