Private file system usage

Hello,
I was wondering if it was possible with out to much modification to make this work on Drupal's built in private file structure?
I was playing around with the code but with my limited knowledge, I couldn't achieve it.
I'm guessing the code around line 19, 27 needs to be changed on audiofield_formatter.inc

...File=' . $base_path . $element['#item']['filepath'] . '"...

Any ideas?

Thanks!