If you can say YES that you have a lg v20 H910 or a lg v20 h918 and the LOS camera on 18.1 records video with normal sound volume, then I owe you my gratification and appreciation and I will be very happy. thanks....Hate to be like this, but you clearly don't understand the issue if you're saying that.
Devices selecting the wrong microphone in the camera app are very much, device specific bugs, that are almost all different.
Some related to microphone input arrays, such as tri-mic setups that aren't being properly utillized, some are incorrect ACDB tables/ID, some are hardware issues where people have stuck paperclips in the microphone thinking it was the SIM tray (WAY more common than your think), some devices have issues with source built audio HALs due to framework side additions by the OEM - there are x^n more possibilities.
And all of them are different.
So, again, if you'd like help on this, I need logs of the _specific_ situation you've hit.