I read mostly with my phone. I use both Kindle and Play Books. Embedded sound (for instance bird songs inside bird watching books) seems to work more often on Play Books, and more often not to work on Kindle. What about HTML? Embeded multi-media always works on a web page.
It is possible to display an entire book on a website using pure HTML and javascript, complete with Next Page and Prev Page buttons. Even with a Last Read Page button, and a searchable index.
HTML as a book format has advantages and disadvantages. What follows is an example, without mentioning the software name or a link to the source codes. So this isn't spam. It's just an example. Why isn't HTML used more often?
Project Gutenberg has complete books online in HTML format. But they come as one giant very looooooooooooooong page. Making a page turning system isn't all that hard, and it makes book navigation an order of magnitude more convenient.
https://montana-riverboats.com/?robo...troduction.htm
The example above does not yet have a searchable index. It will eventually. I'm thinking about how best to make it happen.