Hello All -
I am also experiencing this issue, and am joining the discussion to add to our "strength in numbers."
I received my new iPhone 6s just over a week ago, and noticed the bottom speaker distortion sound from the instant I started using it. The sound is intermittent, but happens more often than not. It's most noticeable with the test messaging conversation sound, as many others have noted. I also experience distortion when my phone rings at full volume, and when using the voiced direction option in maps. I have noticed that if I turn my volume down, the distortion improves, but is still slightly noticeable in some cases. Regardless, I shouldn't have to turn my phone volume down to hide the distortion.
I did have an appointment with the Genius Bar today and was unable to reproduce in store. Just my luck. They ran diagnostics and everything passed. I spoke with the Apple representative about potential causes, mentioning my case and that I had restored the 6s from a previous back up and that I had upgraded from the iPhone 5 to the 6s. He didn't think any of those reasons would cause the sound distortion I was noticing. However, the Apple representative I was working with did offer me a replacement. I have a really nice screen cover and really didn't want to go out and replace it and because I am well within my warranty period I declined to replace the device. The gentleman I was working with told me that I could come back at anytime within my warranty if I changed my mind.
When I got home, I decided to see if anyone else is experiencing distortion, and I came upon this thread. Based on the other replies, I am glad I chose to keep my phone instead of having it replaced. I would hate to have gone through the process only to experience the same issue.
Hopefully, Apple will address this issue soon. While this isn't the end of the world, because of the sound issue, the phone doesn't seem to be worth the money spent. It's really grating to hear the distortion so often.
That being said, I haven't had challenges with any of my other iPhones so I can only hope that Apple resolves this quickly.