a) In most of the cases, it happens because of the AUX cable. Ask customer to ensure if the cable is fitted properly

b) User can use different mobile phone and laptop to ensure if cable is working ok or not

c) For the online videos, this might happen because of the streaming quality or internet speed

d) To some extent, internet can also adversely affect the sound quality

e) If the issue still can't be controlled then arrange AUX cable delivery to customer.