a)  After all the probing questions, please ask customer to use and adjust the earbud in the ideal way.

b) Make sure the volume is adjusted from all possible options. Like- Music streaming APP/Video APP/Mobile Phone and TWS

c) Use the device with different mobile phones and check if a particular song or video has the issue

d) Ask to clean the ear tips in case there is cerumen inside the ear tips

e) In case there is low sound suring call, then there might be network issue or other side user's mobile phone or BT device might have the issue

f) Try to reproduce the issue with different Mobile phones and different APPs