There are three microphones on the iPhone 6. There is one for phone calls/front camera videos, one for noise cancellation and one for the rear camera.
From Apple Support
To test the primary microphone on the bottom of your iPhone, open Voice Memos and tap the record icon. Then speak into the microphone and tap the play icon to play back the recording. You should be able to hear your voice clearly.
If your voice sounds faint when you’re using the speakerphone on a phone call, make sure that your iPhone receiver isn’t blocked or covered.
If audio from video recordings sounds faint, or Siri can’t hear you, make sure that the microphone near the iSight camera—on the top, back corner of your iPhone—and the receiver isn’t blocked or covered.See here for more info.
If the rear microphone isn't obstructed, then you need to order this one.