Super bad audio quality from iPhone 6s Plus speaker
Solution 1:
I saw the same problem with my iPhone 6s Plus. I think it may be an issue with the Skype app since my FaceTime works without any issue.
Solution 2:
Apple generally give 72 hours for a Dead On Arrival (DOA) or Early Life Failure (ELF) swaps.
I would take it to an Apple retail store as soon as possible to have them test it.