He can hear it in his head

Was about to say that

Or he could be looking at it for mistakes, like an editor.

That's.... not really how it works... The sound really depends on what instrument is being played. It's not a "language" more of a "cipher" in the way that you can use it to perform on your instrument.

Except that can be exactly how it works - I sometime just read through the music (on the bus, at home, before rehearsal), maybe humming or tapping along, before playing it. That way I recognise the rythms and have an idea what I'm about to do.

It was said just above that he might be a musician