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 ...
The boom mic is a sensor. It converts audio into an electrical signal. To record that signal you need a recorder of some kind, one that is capable of amplifying a microphone level signal and recording it. Usually that means a digital recording device, which could include something like a Zoom recorder, a camera with an external mic input, a phone with a mic ...
Make sure you do “Download Additional Content” under the Final Cut Pro menu. Download and install if you haven’t.
Left top corner next to the Library Sidebar icon is the Photo and Audio Sidebar Icon. When you in this sidebar you can find “Sound Effects” with lots of sounds you can use for free.