Well, before Overcast appeared I used to just manually process audiobook files with ffmpeg. The command line was:

`ffmpeg -threads 16 -i "$infile" -vn -filter:a "atempo=2.0" -c:a pcm_s16le "$outfile"`;

this produced a .wav file, which is what I fed into Audiobook Builder to produce an .m4b audiobook.

Overcast does a significantly better job, though, and without any extra hassle on my part.