I had an AVI (XVid) file with 2 audio channels. I removed the FIRST audio channel using ffmpeg as:
ffmpeg -i <input.avi> -acodec copy -vcodec copy -map 0:0 -map 0:2 <output.avi>
Now, VLC can play the video, with the correct audio, BUT, no other media player does. I checked with the avidemux and then I found that the audio codec is set as "Unknown".
The original audio codec was AC3/48000Hz and 6 channels. I believe that the only problem is that this information is not written in the file (since VLC identifies the problem and plays the file correctly).
ffmpeg output is:
Input #0, avi, from 'output.avi': Metadata: encoder : Lavf55.12.100 Duration: 01:24:46.68, start: 0.000000, bitrate: 1703 kb/s Stream #0:0: Video: mpeg4 (DX50 / 0x30355844), yuv420p, 720x432 [SAR 1:1 DAR 5:3], 25 fps, 25 tbr, 25 tbn, 30k tbc Stream #0:1: Audio: ac3 (  / 0x2000), 48000 Hz, 5.1(side), fltp, 384 kb/s
How can I fix that?