Hello MajorZer0 ;d
On the iPhone, there's two cameras, two microphones, and two speakers.
There's the microphone and a speaker where your ear goes when talking.
There's also a microphone and a speaker where your mouth is near, at the bottom.
When you use the main camera, the bottom microphone where your mouth is records sound.
When you use the front-facing camera (selfie) the earpiece microphone where your ear goes records sound.
I'd assume most phones go by this concept, and thus.. on your Galaxy's lower microphone module doesn't work.
To confirm my suspicions....
Do a selfie video, say something, and play it back.. if the voice is clear then your earpiece microphone is fine. This earpiece microphone is also used for when loudspeaker is activated.. which is why it works when you put the call on loudspeaker.
If the above is correct, then yes, you have a busted lower microphone and it's in need of a replacement.
There is a chance however it might have something stuck in it and just needs cleaning. ;d