.m4b is the extension for the standard format of audiobooks from Apple. When converting a normal .mp3 in this format you can create audio books with a cover and chapters. And below I will tell you how to make an audiobook.
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Audiobook Binder (macOS)
Audiobook Binder is a free and simple application for macOS, the only purpose of which is to convert .mp3 in the home .m4b.
The app supports adding data about the author, cover art, switching between mono and stereo, change the bitrate, add .the m4b in iTunes library, split large files into smaller ones.
Settings or some magical action for conversion is not necessary. Simply add the file, fill the data and press the Convert button.
AudioBookConverter free utility to convert .mp3 .m4b, but only for Windows.
In fact, SyncMate expert allows converting not only .mp3, but other formats (.wma, .m4a, .flac). And .m4b, you can add the necessary meta-information. The utility can automatically pick up the cover and help set up the announcement.
Application settings helps you manage all the conversion parameters and settings for the output file. Supported multi-thread conversion and the splitting of files into pieces.