since this is happening in variety of apps, and not just one app... this is likely a hardware issue.
Did you shoot pressurized air into the mic holes or the speaker grills trying to clean dust? if so, don't do that in the future. the pressurized air can damage the membrane of the mic and cause mic to fail or distort.
visit an authorized repair center to have the mic part checked and replaced. if it's not a hardware issue then submit error report to Samsung Members after reproducing the issue.
hope this helped.
I'm having the same issue and I see a good deal of others reporting as well on Reddit. Looks like a design flaw, not the user "shooting air into the microphone holes."
im having same problem, but in some apps like call voice not happen, only whatsapp and voice recorder apps, flex mode not work at all and i suppose is last firmware update cause this problem.....