Wonder if the recent stories about the 6S and 6S+ being better sealed against water has anything to do with this. The previous models were not as watertight if at all whereas the new phones have added some seals to protect against moisture. Maybe the speaker has a cover that was not in place on the previous models. Similar to putting a case on like a Lifeproof case. Sacrifice a little sound quality for better protection. I don't guess I have noticed this too much since the speaker on the phones has never been awesome anyhow.