Check How to Use Any Audible Converter on Mac Computer
Run Any Audible Converter on your computer. Click the "Add from local computer" or "Add from Audible library" button to add audiobooks/podcasts to the program. You also can drag and drop the audio-book files into the program window directly.
Click on the "Settings" button on the upper right corner to customize the output quality, and output path, and set output format as "Original/M4A/M4B/MP3".
Click on the "Convert" button to start converting the audio books or podcasts to the output format you chose.
After the conversion is completed, you can click the "History" button to find the well-converted files from the local drive. Now you can enjoy them offline.