What's the 2 mics jobs on the galaxy J7 for?

Noise cancellation. Additional mic for speaker phone. Mic for video recording. Any or all of those...
 
Phones have had 2 or 3 microphones on them depending on model for years.

Simplest explanation is to improve audio quality by having multiple audio sources things like noise cancellation are much easier.
 
It's also so you can get stereo sound when you record video.
 

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