iPhone 11 plays random music without prompting, what's going on?
I would suspect one of two things:
(a) you're accidentially triggering "Hey Siri"
or
(b) you have a hardware problem with the digitizer (i.e. the touch sensitive part of the display).
You can try disabling "Hey Siri" by opening Settings > Siri & Search and then turning off "Listen for 'Hey Siri'".
If you have a hardware problem with the touch input, it will probably need to go in for repair. If you some times in various apps or even on the main home screen experience that "something" moves or taps by itself, this would indicate a touch problem.