From Miles Bader@21:1/5 to All on Sun Sep 27 13:36:11 2015
As Willi mentions, there are fundamental limitations to GIF that make it
hard to get really high-quality output.
It sounds like ffmpeg is not really the best tool to use for this, but
you might be able to make it a bit better by e.g. tweaking the
dithering. You're probably never going to get something really
attractive though.
I did a google search for: ffmpeg options dither -audio