Further to @Superechidna's advice:
The folder accessed via the Twine > Show Library menu items, in your case the documents/twine/stories folder, is the location where the application stores the story Project files and that folder is meant for the application's internal use only. Do Not save the story HTML file you create via the Publish to File option within that folder or you may overwrite and lose your Project file.
The temporary story HTML file created via either the Test or Play options only exists in memory (1), which is why you can't access and display/play an external media file (like an image/video/audio) using a local reference URL like the one in your example.
(1) Depending on the environment you're using a accompanying operating system level temporary file may be created somewhere on your hard-drive(s).