Hi Maxene,
The Story Maps in the Story Maps Gallery listed under 'Story Map App > Custom Designs' are created by our Story Maps dev team as custom developments (not customizations of our out-of-the-box app templates) to experiment with new storytelling patterns or for special one-off assignments. Some of these do get turned into app templates. For example Thirteen Spooky Halloween Haunts was the prototype for the new Side Panel layout we added to Story Map Tour in our April 2018 release (the new Side Panel layout may not have the exact same features as the Halloween Haunts).
In those cases where we have shared the source code for a custom design via Github, this is really aimed at the developer community in case they are interested in how particular features were implemented or have a similar project. We don't provide documentation or support for that code because it is shared 'as-is' and it's not necessarily easy to adapt or re-use the code, and we generally wouldn't recommend that pattern.
In the case of the particular custom design mentioned in this thread, you could get a similar look and feel using the Story Map Tour app and embedding audio. See these examples and here's a blog post about how to do that by including the HTML <audio> tag in the Map Tour caption text.
Rupert
