Generally speaking, if the volume is adjusted on the Apple 6, and the sound is not mute, but low, I think there are two points worth checking.

1 Check the sound hole of your Apple 6 to see if it is blocked by anything. If so, clean it carefully with small tools, brushes, etc. So as to release sound.

When playing games, watching videos, listening to music, or using multimedia software, the voice is low. It depends on the volume settings of the game and software. Find the sound setting and adjust it to the maximum. Give it a try.

3 mobile phone failure, this problem is best to find professional maintenance, if there is warranty, online search how to apply for warranty. Hardware problems require professional maintenance.