AbookioIf you collect your own audiobooks, you will run into four common formats. Abookio plays all of them on iPhone, iPad and Apple Watch.
M4B
The audiobook-specific format. A whole book usually lives in one file, with chapters built in. It is the easiest to manage and the one most players are designed around.
M4A
The same container as M4B with a different extension, common for DRM-free purchases and for single-file books or podcasts.
MP3
The most widely supported format. Books are often split into one file per chapter or part, so you import the folder as one book.
FLAC
Lossless audio. Files are much larger, which matters for storage and Apple Watch downloads. FLAC preserves the audio it was made from; it cannot restore quality lost in an earlier conversion.
Which should you choose?
If you control the format (for example, ripping your own CDs), M4B gives the best experience: one file, chapters and bookmarks. Otherwise keep what you have; Abookio plays all four. Audible and other DRM-protected files can't be played, here is why. For setup steps, see the M4B player guide.