M4B is the audiobook file format. Chapters, bookmarks, resume-where-you-left-off, all the things a plain audio file doesn’t do. Anyone who has organised an audiobook library knows the extension.
That makes this a three-character domain with a built-in audience and unambiguous search intent. A conversion tool, a library manager, an audiobook 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 audiobook market has grown steadily for a decade and the tooling around it remains thin, particularly for independent authors producing their own. There’s room for products, and a name that’s literally the format is a strong front door.
The .net suits a utility or service, which is what most of those products are. It also keeps a three-character domain within reach when the .com equivalent would be out of the question.
Three characters, real currency, fixed supply. Offers considered.