This issue has been discussed previously. It’s an issue unique to the Enhanced mode. The further into the file you get, the longer and longer the resume function takes to start up. There’s also reports that the final 2 - 3 minutes of the every audiobook are dropped compeltely.
The only workaround is to drop back to Mode 4 which offer smaller file sizes and multiple audiobook parts. There’s no issue with Mode 4 playback, except that the Sport doesn’t handle the multiple parts well as it appears to cut off the file name after a certain length. If the Part designation (1 vs 2 vs 3, etc) s after the cutoff, the parts may not be ordered correctly. You’ll need to know the exact length of each part to know you’re in the right order.
What Audible needs to do is create a Mode 5 that has the file size and quality of Mode $, but comes as a single file.
