There is sheet music in the foreground, and it used to be legible. Successive restorations wore the notation down until almost nothing was left of it. What can still be made out on the page is a tenor part and an alto part, and that is all anyone will ever get.
The painting that got him the job
Caravaggio was in his mid-twenties and had just moved into the household of Cardinal Francesco Maria del Monte in Rome, with board, lodging and an allowance. His biographer Giovanni Baglione records that the first thing he painted there was a concert, with some youths portrayed from nature very well.
For three centuries nobody knew which picture that was. The canvas now in New York was identified as Baglione's concert in 1952 by the scholar Denis Mahon, and the Metropolitan Museum bought it the same year. The best current reading puts it before July 1597.
Del Monte was serious about music and owned expensive instruments, which is why the lute, the violin and the recorder in the painting are particular objects rather than props. The cardinal wanted something for the room where music was played, and he got four boys crammed into a space with no depth to it.
Who is in it
The boy at the centre is tuning a lute, head down, both hands busy, and he is the only one doing anything useful. Behind his shoulder another looks straight out of the painting at you — second from the right, and generally taken to be Caravaggio himself, aged about twenty-five.
At the left, with his back half turned and a bare shoulder, a fourth boy reaches for grapes. He has wings. He is Cupid, and he is the reason the picture counts as an allegory rather than a group portrait: music feeds love the way food feeds the body, which was a standard enough idea to put on a wall.
Nobody in it is idealised. They have creased shirts, mismatched costumes half pulled on, and the faces of particular people who were standing in a room in Rome. That was the thing Baglione thought worth noting — portrayed from nature — and it is what Caravaggio sold Rome.
What the restorers took
The canvas has had a hard life. It was in poor condition when it surfaced, and the damage is not spread evenly: it has hit the flattest, most detailed passages hardest, and the sheet of music is the flattest, most detailed passage in the picture.
It was restored again, extensively, in 1983. By then the notation was beyond recovery. A painting whose whole subject is four people about to play something now has a score on which two voice parts remain and the rest is a pale smear.
So what everybody studies is the music that is missing. The lute is still being tuned, which means the picture was never going to show the performance anyway — only the minute before it.
The music in it has been rubbed away by people trying to help. What is left is four boys and a lute being tuned.
Some stories are better over a coffee. Fifteen minutes with someone — audio only, not open yet.