If you decide to protect the audio files, the plugin creates a truncated version of the file to be used as demo and prevent that the original file be copied by unauthorized users.
Verify that the theme used in your website, includes the function wp_footer(); in the template file "footer.php" or the template file "index.php"
Go to the plugins section in WordPress, deactivate the free version of Music Player for WooCommerce, and delete it ( Don't worry, this process don't modify players configured with the free version of the plugin), and finally install and activate the premium version of plugin.
In the pro version of the plugin the files for demo are generated dynamically to prevent the access to the original files.
Each time save the data of a product, the files for demo are deleted and generated again, so, you simply should modify the percentage of the audio file to be used for demo in the product's page.
Note: The above is a safe process, the plugin's data and settings won't be lost during the process and will appear again after installed the pro version.
Alternative Installation Method:
The upgrade instructions are exactly the same instructions mentioned above but be sure to make a backup copy of your current files if you have made custom modifications to the files or translations since the files will be overwritten.
After upgrading the plugin remember to clear your browser cache.
The free version must be deleted before installing the pro version.
If you are upgrading the pro version to a new version you should also remove the previously installed version.
This is a safe step, the plugin's data and settings won't be lost during the process.
Another alternative is to overwrite the plugin files through a FTP connection. This is also a safe step.