Hi @Sigma_nasko, let me see if I can help out here.
The microphone intended for video recording is located at the top of the frame (see point 5 in this overview). Please make sure that this microphone is not covered by a protective case and that the port for the microphone is free from any dirt or lint. You can also test the functionality of the microphones in the support app. Find and tap Settings > Support > Xperia Tests > Speakers & microphones.
Another thing that you can try is to reboot your phone in safe mode and try to record a video there. Safe mode is a troubleshooting mode where all apps downloaded from Play Store is temporarily deactivated and by doing this, you can see if this behaviour is caused by a downloaded app. For more information on safe mode and how to use it, click here.
If the behaviour of the secondary microphone persists in safe mode, I would suggest that you try to repair the software on the phone using Xperia Companion and your device will get the latest update at once. Keep in mind though that if you decide to do a software repair, all user data will be lost in the process, so remember to do a backup before the repair.
Try these measures and let me know how it goes!
