Find the turrets practicing singing in Portal 2

Ellen McLain explained a bit about the creation of the Turret Opera at Anime Midwest 2011:

Video transcription starting from 3:30 up to 4:55...

When we were in the studio, I did all of the recording in a lot of different syllables. You know... Doo-doo-doo, va-va-va, la-la-la... You know, different consonants and different vowel combinations. And then Mike said: "Well, can you just make up some words?" And I said: "Sure!" So I made up words in my very bad high-school Italian, because I studied Italian when I was in the high school. And I have to say that Mike and I have decided that we are not going to divulge exactly what I said, because we love all the creativity. On the Internet, the translations are close or not so close, but none of them have been absolutely correct. But it was, because Mike Morasky told me in the recording session, he said: "This is the goodbye. This is the goodbye to the player. So, hence, this is the goodbye to Chell." So I started making up words in Italian.

Maybe we will never know for sure what are the the correct, "official" lyrics, but we know they are Italian (or something that resembles Italian), and also that some unofficial translations got pretty close.

EDIT: I just noticed that Keaanu had posted a comment linking to Ellen McLain answering questions from Reddit:

5.Q) “What is the story behind the Turret opera song? Was this something you wanted to do with your opera background?”

5.A) The composer, Mike Morasky was the driving force behind the Turret Opera. He wrote all the music. He requested that I use my legit voice (operatic sound) on some of the takes. He chose the takes to use. He asked me to make up some words. So I did so in my bad Italian.

GlaDOS is made up of a few different personalities (the personality spheres), and Caroline was probably one of the ones that was integrated closest to GlaDOS. It's fairly likely that Caroline's more closely integrated and influenced aspects of GlaDOS's development, but GlaDOS clearly has her own personality because otherwise she probably wouldn't have flooded the enrichment centre with a deadly neurotoxin.

There's a much simpler explanation for the turrets ignoring Chell; GlaDOS no longer wants to kill Chell, so the turrets no longer recognise her as a target. Because they're made by Aperture, though, they're quirky enough to recognise Chell as an audience instead. Hence the singing - you can run across them practising earlier in the game.