A few years back I was in the same boat, rocking a Sansa Clip and no smartphone. I'm a little fuzzy on the details, but the way I consumed Audible content was:

- Burning to CD, which their software did - Ripping the CD to wav using other software - (optional) chopping up the .wav files into 5 minute chunks - Compressing everything to MP3 (V8 VBR)

Not sure if they yanked this feature, but worked then!