Sony moeg422 camera model. When I plug in the microphone during an interview, the recording is monophonic. What should I do?

If you mean that only one channel has sound, and you want the other channel to also have sound, it is easy to re-edit it with non-linear editing software and change the audio on the timeline to mono. In this way, the output product will have sound in both the left and right channels. Even without editing, you can have sound from both speakers by selecting the channel in the player. Although this is not stereo sound, for human voices, it does not matter whether it is stereo or mono. If you want to restore the original stereo sound, it will never be possible, because the topic you are recording is mono.