The video file had wrong audio, i could hear only left channel, so i was wondering how to adjust/recode the audio so it is from left+right channel.

This worked to use the left channel also as the right channel:

ffmpeg -i "input.avi" -filter_complex "[0:a]channelsplit=channel_layout=stereo:channels=FL[left]" -map 0:0 -map "[left]" -c:v copy "output.avi"
If you hear only right, try to replace FL by FR and left by right.