Hello,
Are the audiobook files in your iTunes library on the iMac? Since they came from Audible I am thinking they are likely separate files outside of your iTunes Library or the iMac has a different library than your other devices (iCloud/iTunes). If not, reply and we can find different solutions.
Since macOS Sierra was before Apple started replacing the "i" applications it will likely be easier to move the files manually to your laptop before you import them into either Books, iTunes, or Music (depending on what file types they are and how you currently access your audiobooks). Audiobooks bought from Apple would go into the Books App for current reference.
https://support.apple.com/kb/SP742?locale=en_US
If the iMac is "iMac (Early 2009 or newer)" I think AirDrop will be the easiest option. Using Finder, you could select the files then AirDrop them to your laptop (assuming its a Mac) Use AirDrop on your Mac - Apple Support. If your laptop is not from Apple you could use Bluetooth File Exchange instead (assuming it has Bluetooth). Share files between your Mac and Bluetooth devices - Apple Support
After the files are on your laptop you would just need to import them. After they are added to the library they should be included in your current sync.
Import books, audiobooks, or PDFs in Books on Mac - Apple Support
Import music from a computer to Music on Mac - Apple Support
Import media from a computer to iTunes on PC - Apple Support
I hope this helps.