IIRC there ARE two microphones on the S6... One that works for normal voice calls and one that works when the speakerphone and audio/video recording is active. If the 'normal' microphone is broken, you should still be able to make calls via speakerphone (or viceversa).
Use a strong light to inspect the mic hole on the bottom. If there's lint or dust in there, it's a good sound absorber, and it can make it sound like the mic is broken. Clean the hole out with a thin wooden toothpick.