Why is the iphone6s SMS voice too low?

Whether you are playing games or listening to music, you should turn down the volume of the media. Just turn on a game or music player, and then press the Volume+button to turn the volume back, and it's OK.

Because the volume is divided into ringtone volume, call volume and media playback volume, 6S puts notification ringtones such as short messages together with the media playback volume, so if it is turned off, the notification ringtone will not be heard.