If you have a dual audio AVI, you can use
avidemux. Open your file, let it make a VBR map/unpack bitstream if necessary, go to Audio in the tool bar at the top, select the audio stream you want (use VLC to determine which if you're unsure) and then save the file.
I don't have a dual audio file to hand to confirm but I'm pretty certain this was the method I used to whack an unwanted German stream once.