ffmpeg take channels 7+8 and make a stereo
Try something like this:

1) Merge Audio Tracks 7 and 8, put it in Audio Track 1, discard other Audio Tracks from source:

ffmpeg -i "in.mxf" -filter_complex "[0:7][0:8]amerge=inputs=2[mergedAudio]" -map 0:v -map "[mergedAudio]" "out.mp4"


In some cases you might add the option -ac 2 between the end-" of filter complex and the 1st map.

2) 1) combined with all (useful) parameters of your code:

ffmpeg -i "in.mov" -filter_complex "[0:7][0:8]amerge=inputs=2[mergedAudio]" -map 0:v -map "[mergedAudio]" -ac 2 -c:a aac -b:a 320k -ar 48000 -disposition:a:0 default -strict -2 -c:v libx264 -b:v 9300k -minrate 9300k -maxrate 9300k -bufsize 2325k -r 24 -force_fps -s 1920x1080 -aspect 1:1 -pix_fmt yuv420p -profile:v high -preset medium -level 40 -partitions partb8x8+partp4x4+partp8x8+parti8x8 -b-pyramid 2 -weightb 1 -8x8dct 1 -fast-pskip 1 -direct-pred 1 -x264-params force-cfr=1 -coder ac -trellis 1 -me_method hex -flags +loop -sc_threshold 40 -keyint_min 24 -g 240 -qmin 3 -qmax 51 -threads 12 -metadata creation_time=now -sn -y FINAL.mp4


You had a space in your bitrate -ab:1 320k. I have changed it to the current syntax -b:a 320k

Same with -ar:1 48000 -> -ar 48000

Removed -async 1 because it threw an error