iPhone 4s microphone problem

I have an iPhone 4s, and for some reason, only one of the microphones work. The microphone for talking during calls, creating voice memos, and talking to Siri works great, but the mic for speakerphone or video recording doesn't seem to hear anything. Although if I turn the volume all the way up and listen VERY intently, I can very quietly hear the audio that it was supposed to be recording. I'm assuming that either the microphone has something blocking the sound, thus it being unable to get to it, or the microphone is just plain broken.

Any thoughts onto getting this issue resolved would be greatly appreciated!