afconvert -f caff -d LEI16@22100 -c 1 ./test.mp3 ./test.caf
I kept reducing the sampling rate from 44100 to 13100. Anything below 22100 seems to affect the quality of the audio.
You can also convert a caf file with a given sampling rate to another caf file with different sampling rate:
afconvert -f caff -d LEI16@22100 -c 1 ./test.caf ./test2.caf