a) Ensure volume is set to maximum through Mobile Phone and ear buds both

b) Use suitable size of eartips
 

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

d) Ensure cleaning of eartips. Ask customer to clean it carefully and validate this by image
 

e) Please make sure if customer is facing this problem with any specific song/video or APP

You can different ear tips in both ear buds as per its convenience. Like- "S" in right or "M" in left vice versa and so on

 Please check of either side of ear buds fitment is not proper, means "L" side is properly fit and "R" side is not fitted correctly and vice versa

 Please do not use any sharp object to clean ear tips. It may damage the sound mesh