ffmpeg how do i know what audio rate to use?

Say I have something like this

ffmpeg -i video.avi -ar 22050 -ab 32 -f flv -s 320x240 video.flv 


-ar  (Audio sampling rate in Hz)
-ab  (Audio bit rate in kbit/s)

regarding the -ar and the -ab how do I know what rate to use? I got this ffmpeg command from a site somewhere and I was wondering how the person knew what values to put for the rates? Do I need to understand audio in order to figure that out?