So you think it might be converting the files just to apply the volume leveling? I'll have a look at that. And MMW says is it converting the files when I sync them for the first time, and they lose their chapter markers.

You may want to disable Volume Leveling (not sure it applies to M4B though). How are you verifying that the files are being converted?

Really sorry, I thought I would have gotten a notification when there was a reply.

Anyway, I should have been clearer, but M4B to M4B. I'm on 4.1.21.1875, but about to download the new version. And the only auto conversion rule is incompatible audio to MP3, so I think that means the issue isn't there. Any thoughts?

So, it's converting MP3 to M4B or M4B to M4B? Which version of MediaMonkey and did you also check the Auto-Conversion settings besides the Supported Formats?

I should have said it converts them to M4B again...

I've created some audiobooks from MP3s and added them to Mediamonkey, but when I sync my devices (a USB and a Note9), Mediamonkey converts them. After I first noticed this I changed the 'Supported formats on device' settings to include M4B, and ticked all the boxes in 'Supported format configuration', to try and ensure it would work, but still it converts them. Apart from the time this takes, it removes the chapter markers. So is there something else I can do to stop Mediamonkey from converting the files when I sync?