I stumbled across a few previous discussions regarding this problem, but the threads dropped before catharsis.
I have a t6i that I've used for video just about every day since February. I mounted an Opteka stereo mic on hot shoe and it worked fine until the other day, when I realized that I could barely hear any sound, and it was crackly.
I checked the camera's own internal mics and they work perfectly.
I checked the mic, took it apart, and resoldered a wire or two, thinking that was it, but It didn't change the result.
I then hooked the mic up to my laptop and the mic is perfect.
I checked the recording levels in the camera, and it looks like the sound is fine (maxing out with appropriately loud noise, etc.).
When I record, though, the same dim, crackling sound is the result.
It is extremely frustrating to watch the levels dance on the screen, but not have it record on the card.
Based on what you describe there are two things I would check. First I would just check with a small light to confirm here isnt significant deris in the mic port. Anything like pocket lint or i've even seena small piece of a gum wrapper get in there and black some connection. If the port itself seems clean the next best thing to check would be another microphone designed for DSLR cameras if you have access to one either by a friend or a demo model at your local shop. This would hepl confirm if the issue is internal to the camera or not.