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Unfortunately, you are stuck with the in-camera mic with the T6. The T6i has a microphone jack for mounting an external microphone. If you want to mount an external mic to a Canon Rebel T6 body and record that audio stream to the same SD card, that's really your only option.
Can you ...
First, have your subject(s) rehearse the heck out of their presentation if that is practical. If you have access to a Mac, place it near a wall, preferably white. Bounce a cheap lamp off the wall to give your subject soft diffused light, and record using Quicktime (it's free). You can do quick edits right inside Quicktime if you need to.
What I would do is concentrate on the setup in front of the camera, to make sure that what you're recording is the best quality it can be, in order to avoid the need for post.
Set up your lights so that you have good, soft-ish (no harsh shadows) lighting that is well colour balanced. A 3-point light setup is the standard way of lighting interview subjects, ...
After searching forums and YouTube videos, I believe I've found the culprit:
My building's ultrasonic motion detectors.
They emit an ultrasonic chirp every few seconds that the mics 'hear' and turn the volume down (attenuate?) for that portion.
I've contacted Sony Professional Equipment support and I've been issued a Beta Firmware upgrade for the ...