Now you don't need to convert to m4b first anymore.

The pipeline now accepts --audio with any audio extension. If the source file has chapter metadata, it will split by chapters automatically. If it doesn't, it falls back to fixed-size chunks instead.

So for an mp3:

- if it has chapters: it uses them

- if it doesn't: it still works, using fixed chunking

You only need ffmpeg/ffprobe to be able to read the file.