Where does $audio_file get its source

In the TPL files, the audio player seems to be populated by this line of code:

<a href="<?php print $audio_file; ?>"><?php print $audio_title; ?></a>

I'm wondering where in the code the $audio_file variable pulls the URL to embed via href in the code. In short, I'm wanting to change the source so I can print the URL from a text field into the $audio_file variable. So, instead of pulling the $audio_file URL from the uploaded file, it populates it from "textbox1" field.