m4b.net
Chapters, bookmarks, resume-where-you-left-off. M4B is the audiobook container format, and anyone who has organised a library knows the extension on sight.
That gives this three-character domain a built-in audience with unambiguous search intent. A conversion tool, a library manager, a player, a self-publishing service, or a podcast-to-audiobook product would all be found by people who already know what the letters mean.
The market supports it. Audiobooks have grown steadily for a decade while the tooling around them stayed thin, particularly for independent authors producing their own, which leaves room for products and a strong front door for one.
The .net suits a utility or service, which is what most of those products are, and keeps a three-character domain within reach when the .com would not be.
Three characters, real currency, fixed supply. Offers considered.