Yes it is 3x louder (Apple confirmed it when the Pro 12.9" came out) Plus having owned all iPads and iPad Pros, (and I still have the 2010 original, Mini 4, Air 2 and Pro 12.9").... The 12.9" iPad Pro's sound is unequivocally louder, clearer, has much better bass and greater sound equalization/balance than any other iPad (especially the Air 1 and 2.) I've yet to see a tablet that rivals it's sound. It's even louder than my laptop and can be heard over my TV until I blast the surround sound system to about 21 out of it's 30 max.
I work at Best Buy and have found the same goes with the displays we have when I test them out, I'd say you have a faulty iPad Pro... well first before I say that, I'll ask: are you listening to the tablets separately or at the same time? (Usually one can tell the difference without having to compare side by side,) but not everyone's hearing is the same. I recommend comparing side by side and if the sound is still not loud, take it to Apple. *Not trying to sound like a jerk or know-it-all, but It's really simple, the iPad Pro 12.9" has 3x the sound output of the Air 2 because it has 3 more speakers installed (and each one is the same size as the Air 2's.) The Pro 9.7 is about 2.5x louder than the Air 2. But again, compare one more time... if no luck, take it back, as the Pros speakers are one of the biggest features Apple touted and highlighted as being extremely loud and robust, and I agree wholeheartedly with them, (for once.)
Kallum.