My iPhone will not play sound through lightning/USB cable to Mac Mini running Mojave

I have a music App on iPhone which I want it to play through my Mac Mini with the following setup

iPhoneX(iOS12.3) --Lightning/USB Cable ---Mac Mini(Mojave10.14.5)--Headphone Jack --Speakers

(I could physically swap cables between Mac and iPhone but I don't want to do that.)

I enabled the iPhone through the Audio Midi setup app in utilities.

I designated the iPhone as the input device in the Sound system settings.

I can see the input level moving up and down in the sound settings when I play sound on the iPhone so it seems OS X is recognizing some signal.

However, I can't hear any sound from iPhone through the speakers.

I can hear sound from iTunes on the Mac so the output settings on the Mac seem OK.

Is this an OS X bug? Any tips are appreciated.