Thanks for the suggestion, Jason,?
When I checked the sound card settings in WSJTX after plugging in the second monitor and rebooting - the sound card disappeared and was no longer an option in the selection list and the original sound card entries for microphone and speaker were showing not found (see picture), I went back to a single monitor and the sound card reappeared.
I then tried to overcome the problem with the status window in Build-a-Pi which was no longer transparent when using two monitors - please see the attached photo. I thought that by swapping it might correct this problem, so I swapped the monitor connections to the HDMI ports so that the monitor connected to HDMI-1 was now connected to HDMI-2 and vice versa. On reboot the sound card was now recognized and WSJTX works :) I can't explain why but if this worked for me then maybe someone else can benefit from trying the same.
BTW is there an easy way to change your callsign on the status panel as mine has changed to my US call since I originally installed build-a-Pi.
Thanks again and great work.
Richard W5ARH