a) Ensure Mobile phone and BT device should be within Bluetooth Range

b) If music stream is online, make sure internet connection is stable

c) Listen the same song either with other Neckband or through different Mobile Phone

d) Select best streaming quality for better sound experience

e) Ensure if distortion is subjective or actually some hiss or distorted sound is there. 

Steps to reproduce solution 

Keep both the device in BT range

Ensure good internet connectivity

If possible, use other BT device if he/she has. If don't have then they can use other Mobile phone with same BT device

Open Music APP setting and set the streaming quality to maximum