how to properly include or get file contents in a wordpress theme

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There are 2 problems:

include( get_template_directory() . '/audiounity/result.php' );

The result of this is likely:

/srv/www/example.com/public_html/wp-content/themes/audiounity/audiounity/result.php

The first line of the Codex entry for that function is:

Retrieves the absolute path to the directory of the current theme.

If it’s a child theme, then you need to use get_stylesheet_directory. *_template_* functions always refer to the parent theme, *_stylesheet_* functions refer to the active theme

A Bigger Problem You’ve Missed

$data_file = unserialize(base64_decode(file_get_contents(' get_template_directory() . "/audiounity/website_setting.conf"')));

  • .conf files will be readable to all, anybody can download your themes .conf file, would it not make more sense to store it inside a PHP file as a PHP array?
  • unserialize will unserialize a PHP structure, which exposes you to a PHP object injection attack. Avoid serialising and deserialising PHP

If you want to store a configuration, you have 2 options:

  • use JSON, json_encode and json_decode
  • use a PHP array in a file

If you need to modify these files, don’t, use them as defaults and store the changes in the database as options