Apple iTunes recognizes and categorizes MP4 audio files (preferably with an .m4b extension, not .m4a nor .mp4) correctly as audiobooks if the tag field stik MediaType is set to 2 for Audiobook. I shall assume that its successors, i. e. Music, Podcasts and TV, do so as well.
Classic MP3 files, however, use a different metadata tag format: ID3, which has no official, documented way to mark a file as an audiobook. (One can use any string for TCON content type, usually labeled genre, but that does not seem to matter to iTunes.) The kind of audio can manually be set in the iTunes GUI, but this will apparently not add or change the ID3 tag in any way, instead just update the database entry and possibly file location. This has been documented many times on the web, as has converting the MP3 into an AAC MP4.
- Is there any way to tag an MP3 file in such a way that Apple audio players/managers will import it as an audiobook without manual correction?
As an alternative, Iʼm considering to put a remuxed MP3 audio stream (i.e. not reencoded with AAC) into an MP4 container (v2, i.e. MPEG-4 Part 14, .m4b file extension), but I donʼt know yet whether Apple supports them correctly.
MP3s can also use different tag formats, e.g. APE from Monkey's Audio, but as far as I know none are recognized by iTunes.
Best Answer
No, there is no documented way to mark MP3 files with ID3v2 tag as audiobooks in a way that Apple products (or any hardware or software player really) would understand. There apparently is also no undocumented way known either.
Although there is a public Github repository, the development of ID3v2 has stalled for over a decade now and should not be expected to be picked up ever again, because more recent container formats, like MOV-based Part-14 MP4, use different metadata formats.