a)Check the ear cavity seal, suggest using correct eartips to get best seal. 
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 an issue.
d) Clean the ear tips in case there is cerumen inside the ear tips.
e) In case there is low sound during a call, then there might be network issue or other side user's mobile phone or BT device might have the issue.