It's especially necessary for standalone Watch playback, as there's very
little access anywhere in watchOS to the concept of the system volume level.
Why bother with these
little cartridges is what I wonder, enable the text on a website with the links embedded, make the enhancements available online too and charge for
access to the bundle, update it when new books are released and cross sell products if you want but crucially make it available to anyone willing to pay on any device
anywhere that's connected to the web.