This could be a replacement parts quality issue. The button flexes could be defective or damaged (they are easy to damage). Hopefully you haven’t sealed the device before doing final test.
You’ll want to remove the [product|IF276-003|Volume button flex] and inspect it closely for tears or connector damage. I would do the same for the Power button flex.
As for the initial reason you did the repair, did the microphones fail out of the blue or was it following a drop, repair attempt or contact with water?