ffmpeg - copying original audio to second audio track, then applying filter to only first audio track
Map the input audio twice, and set codec option for 2nd audio track to copy.
ffmpeg -i "input.avi" -map 0:v -map 0:a -map 0:a -filter:a:0 "channelsplit=channel_layout=stereo:channels=FL,volume=+16.2dB" -c:v ffv1 -coder 1 -context 1 -g 1 -slices 24 -slicecrc 1 -c:a:1 copy "output.mkv"