I have connected my Teensy 4.0 to an Audio shield Revision D
and I get no headphone output using the recommended guitar test.

The Audio shield is mounted on top of the Teensy 4 using female headers
pointing down onto the pins soldered to the Teensy 4 . I have checked the
continuity from below the pins on the Teensy to the top surface of the
Audio shield and all conduct. (This way if more connections are required then using
the female headers it allows me to separate the 2 boards and connect extra pins.)
N.B. Only the 16 pins given in the diagram below have been implemented.
Is that correct ?
Without the Audio shield the Teensy works perfectly accepting many
of my previous C programs and the examples e.g. Blink and uses the
serial channel.
With the shield the guitar sketch downloads but when I open the
serial channel in Tools it fails not recognising the serial channel.

I believe my soldering to be ok and I have resoldered any junctions that
could be dry.

Can any one help ?

Click image for larger version. 

Name:	teensy4_audio_pins.jpg 
Views:	11 
Size:	69.4 KB 
ID:	17907
Click image for larger version. 

Name:	0[2].jpg 
Views:	25 
Size:	125.8 KB 
ID:	17906Click image for larger version. 

Name:	0[3].jpg 
Views:	32 
Size:	62.4 KB 
ID:	17908Click image for larger version. 

Name:	0[1].jpg 
Views:	27 
Size:	137.4 KB 
ID:	17909