every time i want to make a kvcd of an avi file where i disable audio encoding ffmepgx still encodes the audio ....why does it happen?

and why ffmpegx reencodes the audio through qt "audiodump.wav" it costs much time .. it can encode audio mp3 to audio mp2 in few minutes when i choose it in quick present ,why ffmpegx didint demuxx the avi file and then encodes the both files seperly without "audiodump.wav"

thx
jt77