After importing videos, there is always at least one that shows no audio/ has mixed audio

It's been a couple of months that after importing video files from sd card I get always one or two whose audio doesn't show on the timeline, even though I can hear it if I play it on the timeline.

Furthermore the 4 audio channels are all mixed in one single audio and I cannot de-select the ones I don't need.

Here you can see how it should be.