I never use in-device speakers. On TV's phones, laptops and tablets they're all terrible. I bought a decent Sony BT speaker for £60 - best accessory I have bought for my iDevices.
My tablet sounds rubbish and previous phones have sounded rubbish. The 6+ was uncannily good for listening to music though, so why should Apple considerably downgrade the 6S+ loudspeaker? I was expecting exactly the same speaker.