“Hey Siri” does not respond while playing music It seems it’s impossible to wake Siri up using the “Hey Siri” functionality while playing music (using either internal speakers and headphones.) 

Is there a way to solve this issue?
As far as I'm aware Hey Siri isn't actually supported while music is playing.

People on Reddit have reported the same issue about a year ago - https://www.reddit.com/r/iphone/comments/7ssnz7/hey_siri_not_working_when_music_is_playing/