Sometimes I turn captions on when I'm watching stuff on Netflix. This is partially because I like to do other things while I watch stuff. If I miss a line, I can just take a look at the script on the screen.
I've noticed something.
Usually, a caption will indicate the presence of music in a scene with a simple phrase like "music playing". I'm pretty sure that I've even seen captions that just throw a few quarter ntoes up on the screen to indicate this. Occasionally, the actual song will be stated. I just can't imagine a purpose that would be adequately served by this. Is a person with congenital deafness going to know what Fatboy Slim's "Praise You" sounds like? He might be vaguely familiar with the general concept of music, but I'm skeptical about his abilities to differentiate between the thematical forces imparted upon a film by an alternative rock song and a dubstep track.
This practice might be slightly more helpful to an individual who lost his hearing at a later point in life, but I've got to think that it would mainly just be depressing for him.
"Aw, man. I remember 'Praise You'. That was my jam in the Nineties! I remember the Nineties. Great times. Hot tunes were on the radio! 'Austin Powers' was in theatres! And I could hear stuff. Damn."
Best subtitle? "The True Story of a "Most Objectionable Nazi" and... half-a-dozen Cats".