Same issue here - no case in the way, just switching to full volume on the external speaker and the distortion is audible when impulses are to much for the external speaker that is build in.
Quote from taliota24 :"I'm referring to the speaker on the bottom of the phone, not the ear speaker used when talking to someone. Anyway I noticed that when the volume is high, about 90% of the max or more, I heard an intermittent light crackling or popping sound when playing music, watching youtube, or using the speakerphone. Has anyone else noticed this, or is my speaker defective somehow?"
Also I got my iphone 6plus last week. and because of the distortion issue I got today an exchange warranty iphone 6plus
The problem stays , so hopefully Apple looks into this, what happens with audiophile material on full volume in their hardware construction of the ext. speaker.
My opinion is not out of boredom or bashing anyone!!!
- to me the iphones are great tools to demo music examples at shows - I run an Audio company and have the same music library on ipad, iphone5 and even iphone 4 .
Some of the music is mp3 most of it is higher data quality also I have Spotify premium and I have now created a file saying:" distortion examples".
NONE of the other apple devices I have produces any distortion at all.
It can easily be reproduced for example with the following track from Melody Gardot " So Long" - it starts right at the guitar intro.
So in case one wants an example to see if it is a general problem on the new hardware
Thanks for starting this theme and thanks to those who look competently into it.
Martina