a) Ensure volume is set to maximum through Mobile Phone itself

b) Please use suitable size of eartips and wingtips

c) Please use other music APP/music player as different songs/videos app have different audio quality with different Mobile Phones

d) Please play same song/video with different APP and Mobile Phone

e) Ensure cleaning of eartips and validate this by image and ask customer to clean it carefully

Steps to reproduce solution 

Control the volume from Mobile Phone

You can find different size of eartips and wingtips in accessories box

Different music APP customer can download from APP Store

Different APP has different quality of music

Do not use any sharp object to clean ear tips