Combining video files with different number of audio channels

[midasthegod]

I'm trying to combine 3 different video files and reencoding it into 1 video file.
2 of the files have six audio channels and 1 of them is stereo file.
If I reencode it into 1 video i always have a terrible screeching sound during the clip with stereo audio. How can I avoid that?
Here are the 3 video files that I want to combine:
https://drive.switch.ch/index.php/s/uNoMU7rqwUv7opb

Any help will be greatly appreciated.

[eumagga0x2a]

You have the choice either to downmix and re-encode the audio track of videos 1 and 3 to stereo or upmix and re-encode the audio track of video 2 to 5.1 channels (which won't remain unnoticed with a proper audio equipment) prior to appending 2 to 1 and 3 to the both. After this step, it should be possible to save the result in copy mode for the audio track.

When appending videos, all properties for all the audio tracks must match.

[midasthegod]

Perfect, I think I know what do do now. Thanks!