I had been trying to wrap my head around how I might be able to monitor the audio going into my Canon T6i Camera. The camera only has a mic input and no headphone jack. I recently discovered the COMICA CVM-AX1 as the solution I needed. This camera audio mixer allows 2 channel input and a headphone jack to monitor the levels of the audio going into the camera.
It is powered by 2 AAA batteries. This really opens up some options for my youtube channel. I can connect a shotgun mic and a lavalier mic with one going to the Left Channel and the other going to the Right Channel. I would then select Stereo if I'd like to seperate when editing the videos. You could also plug in two shot gun mics and have them set to two different volumes, that way if the sound gets too loud then you have a backup plan to get audio that hasn't peaked yet.
Another benefit is that there are 3 cold shoe points on the audio mixer. This allowed me to mount a shotgun mic on the side and my LED light on the top.
I hope this article helped shed some light on the options you have for monitoring your canon camera especially the Canon T6i.