I just went through two Air 2s with different issues week before mini 4 came out. First one had bad screen with thick darkening around borders of display kind of like book spine thing. That one had speakers that got super loud and vibrated the air 2 in your hand like a vibrating bed.
Second air 2 looked fine at the store and speakers were less louder, as compared to other one and screen looked ok. Got home in a normal light condition in room and notice in portrait half left of screen yellow "warmer" color other side of screen from center to right blue white. So very bad uneven screen on solid colors. This air 2 also vibrated with speakers but it was not as bad as the first one I tried. With volume 20-30 percent it was manageable.
I wanted to keep that one but the screen being yellow tinted on half of it and white on other was not fun to look at compared to my air 1 having a good white color brightness even across the whole screen.
Returned them, found out mini 4 was coming and got one. Screen is amazing compared to air 1 I own and the two defective out of box Air 2's.
Volume at max on mini four is a tad louder than my mini 2. At max volume speakers distort kind of but there is absolutely no vibration from speakers shaking the housing or frame of tablet in hand.
Compared to my mini 2, quality of audio is a tad worse instead of better than my mini 2.
My mini two has amazing little speakers, and from others who have heard them, they cannot believe the sound quality, bass, and lows, mids, and highs are distinguishable. They have a full dynamic range.
Mini 4 speakers sound good but are yet different from the mini 2, air 1 and compared to an air 2 way better.
I got over this about 30 seconds after getting my Air 2 last year....so did everyone else.