You can set the URL to audio file directly in "Video set source" action, without AJAX. It works for me. For example:
Ogg Theora Source="https://file-examples.com/wp-content/uploads/2017/11/file_example_OOG_1MG.ogg"
Also, you can play audio files by names if you upload them into the folder with your game. You can do this after you exported you game, no need to include these files into the project.