I thought so also; but I kind of remember that the speaker was improved with the 5S. Since we don't have a 5S, I can just compare to our 5; and the 6 is much better.
Not sure about the 6, but they sure as hell did with the 6+. This thing is much louder than the 5S yet crystal clear at max. It's really one of my favorite improvements.
My wife got her 6 on Friday. The speaker blows away the one on my old 5. Looking forward to seeing if the 6+ that is due in tomorrow is even better than the 6. Either way, I am happy that the audio is now quite adequate with the 6 series.