Video Microphone Buzzing after Repair?

I recently followed the iFixit guide to replace a smashed front assembly on my iPhone 4S. All went well and my screen is now fine thankfully, and the phone functions fully... except for a buzzing interference noise instead of audio on any video I shoot with the phone. All other audio input is fine, voice calls, Siri, voice memos etc, but video recording produces this humming audio. I'm guessing I didn't ground / correctly reconnect something somewhere, so does anyone have any idea on the specific area of the iPhone where this problem would occur? Ideally I'd like to whip it open and fiddle about with whatever it is that handles the audio for videos and see if it works again but I have no idea where that microphone / circuitry is within the phone. Thanks very much in advance for your help. :)