I think it might have something to do with the fact that occasionally one's singing ability is considered a replacement for a character trait - basically instead of describing personality or whatever you get 'good singer!'. So singing ability with other character traits is well and good, but the annoyingness crops up when it's their only character trait.

It might also have something to do with descriptions. If someone has a voice like bells or a flute or whatever, it's not fun to hear that noise distorted into English words.

In that it's a combination between overall speshulness and Aura of Smooth.

The Aura of Smooth effect would probably manifest in the slavish devotion that canon characters suddenly manifest in the Sue after hearing her sing, which ties into the "singing is speshul" option very effectively.

In some cases, the psychic Sue-song can even cause possession in the canons, and I've found one badfic where a character was completely replaced after hearing it. or, if they aren't posessed, the singing tends to be chgaracterized by urple prose or Suvian traits in canons. Seeing someone shed a Single Tear at hearing the beautiful voice of Mary Sue is something that I have seen so many times, and it gets really annoying.

Some Sues tend to attach in-story powers to their singing as well for even more speshulness, usually "graceful" side effects like "curing evil" or "causing people to fall in love". The problem is, that's a mind-altering psychic power that most Agents need Glopsnerch or some sort of anti-psionics training to fend off properly. Sues and psionics are a bad combination, since Sues have little intelligence but a willpower strong enough to dominate an entire world.

It made me a little uncomfortable, because I take pride in my singing voice (I'm not great, but I can carry a tune without hurting people's ears) and don't like to see anyone mocked for being a good singer.

However, after giving it some thought, I've come to a few conclusions.

1. Sue-singing invariably sounds bad to anyone not affected by Aura of Smooth, a.k.a PPC agents. It's only said Aura of Smooth that makes the canons think it's pretty.

2. The Sue almost always uses it as some speshul trait, flaunting it and being complimented and fawned over by the canon characters for it.

I think that should be specified in missions, especially the first point. Otherwise, it comes across as "good singing = Suvian and annoying", which is not true.

Or at least their Aura makes people think she's good at it. To some agents she just sounds like an awful banshee.

Also she often sings songs inappropriate to the setting - one of the TOS Sues sang Nickelback and Alien Ant Farm songs in Moddle-earth, if I recall.