As a standard RTF editor, Scrivener doesn’t support inserting multimedia files into a text editor, so that part of what you are describing isn’t something you can do directly. And while it is possible to insert your own custom HTML through a variety of different mechanisms, in this particular case you would still need to do some work on the .epub file after compiling it, in order to get the audio files properly addressed in the manifest. So overall I would recommend just putting a text placeholder indicating which file should go where, and save inserting them as the final production step using a tool like Calibre’s editor.