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Switch channels of stereo sound
Switch the front left (FL) and front right (FR) channels of a stereo sound.
ffmpeg \ -i input_file \ -map_channel 0.a.1 \ -map_channel 0.a.0 \ output_file
The sound channels are mapped in the inverted order, therefore they are written in the inverted order into the output file. As usual in IT, the numbering starts at 0, not at 1 («zero-based numbering»).