How to access files over htdocs with javascript

I'm actually coding a website to access my data stored on my NAS.
This NAS is also the host.

So my folders seems like:

/data/music  
/data/images   
/www/index.php  
/www/script.js

I made a file explorer with ajax requests and php,
and I want to create a mp3 player which play when we click on a mp3 file, in the file explorer.

So my question is,

How to accede the mp3 file stored outside the www folder, with a client side language ?