I'm using an external wireless lavaliere mic with my Canon T4i, but I'm still getting sound from my built in mic. How do I disable only the internal mic when shooting video?
The internal mic should be disabled when the external audio source is plugged into the microphone jack on the side of the camera. I would first check to be sure the 3.5mm mini plug from your external microphone is securely plugged all the way in.
If that doesn't solve the problem, there may be an issue with the impedance and/or voltage output of your wireless receiving unit. If the 3.5mm jack form your receiving unit is mono that may also cause a problem since the input jack to the camera is stereo. Try plugging in a straight hot-shoe mount type microphone (preferably one with a longish cord so you can isolate it from the internal mic) to see if the camera still records from the internal mic.
Based on the notes on page 194 of the T4i Instruction Manual, if you are shooting in a Basic Zone mode (Scene Intelligent Auto, Creative Auto, Portrait, Lanscape, Close-up. Sports, etc.), you may need to change to one of the Creative Zone modes (P, Tv, Av, M) to disable the built in microphone.