Need help translating js with closures and callbacks to WebCore

I'm working with the Web Audio API and I need to load some audio files from both local storage as well as a URL into buffers and then play them. There are lots of examples of this in js, and they're all pretty much the same. I'm just unclear how to implement them in WebCore. Here's something that has a nice explanation:

At the end of the first paragraph there is a link (BufferLoader class) to a short file that is essentially what I'd like to have implemented in WebCore, although I'm not clear if it supports local file access:

https://www.html5rocks.com/en/tutorials/webaudio/intro/js/buffer-loader.js

There are several closures and callbacks, and I'm not sure how these would translate into Delphi to go through the pas2js layer.

FYI: I plan to load a bunch of buffers up first, then copy them into a larger buffer and then play them later -- ie., O(minutes).

This looks like it may be solved by upcoming async() features added to Webcore 1.7, yes?