Hello:
I see you have a video file in an MKV container. Maybe you have several audio tracks inside the video.
I do some recording for a course (I use OBS Studio) and set up a video with 3 audio tracks, where the first track is the desktop audio, the second track is just the electric bass input, and the third track I reserve for the webcam microphone.
The resulting recording is an MKV video with three audio tracks.
So when I import into Shotcut, I can choose which audio track I’m interested in.
In the example, I have added the same video to the three tracks V1, V2 and V3. Then in “properties” in the “audio” tab I can choose the audio track.
I don’t know if this will help with your problem.