I didn't hear anything about improved sound, but coming over from the 5 I notice the vibrate and sound seems a lot louder. It could be the design but at max volume its real loud whereas before max on my old phones I could barely hear them at times.
The vibration on my 6 Plus is noticeably loud. Sometimes I think it actually drowns out the ringtone or text sounds. I've seen some people complaining about this on other forums.
I really like the phone so far, so its not a deal breaker for me but I hope maybe Apple can implement a software update to adjust the vibration motors' strength.
Do you guys remember how smooth the vibrate was on the 4S? Then they changed it to the "old" style rougher sound on the 5 moving forward because it took up less space or something.