Make sure your headset and the mobile are within 10 meters range.

If customer streams the music Online make sure the internet connection is stable

Select the streaming quality to the maximum for better sound quality, If this doesn't help proceed with replacement as per     policy

7.1 Check for the streaming quality 

On mobile tap on the Music player

Tap Settings

Tap Streaming Quality    

Select HD or the High

1. Distortion on  Music or calling : calling : Normal or Online : Normal call : Network issue

Wi-Fi calling off if still issue persist go for Reset and if not resolved go for RPU.

Music Online : Change network/ Wi-Fi and check still issue persist check streaming quality and Reset.