I have just upgraded from an T6 to T6i specifically to be able to use an external microphone for video.
The camera is recording sound fine with the in-built mic, but when I plug in the external mic (a lavalier type lapel microphone) it won't record a thing!
I know the mic works, as I've tried it with my laptop.
EDIT: I discovered the reason. The microphone has a TRRS plug and the camera only accepts the TRS plug (I didn't even know there was a difference!)
Fortunately, you can just buy a TRRS (female) to TRS (male) adapter. Problem solved.