No Audio through USB-C to HDMI on 2018 Macbook Airs.

I work at an educatonal institution, and we are having an issue with some of the newer 2018 13" Macbook Airs.

2 out of 3 of the MacBook Air's will not output audio to HDMI via a USB-C to HDMI adaptor.

The 3rd Air works fine however.

HDMI output simply does not appear in the list of Audio output devices in System Prefs.

Tried multiple HDMI screens, multiple different HDMI cables and 3 different USB-C to HDMI adaptors.

a. A generic cheapo brand

b. HP USB-C to HDMI 2.0 adaptor.

c. Official Apple USB-C to multiport adaptor (USB-C, A and HDMI) Model: A1621.

All 3 Air's are running the latest OSX 10.14.3, Local account is Admin and have Similar software suites installed - A fairly standard collection of MS Office 2016 and Adobe CC.

Reboots did not help, nor did swapping around the USB-C port use.

Out of idea's as what to try.

Cannot see anything obvious as to why 2 Air's don't work, yet the 3rd is fine.

PC's and Earlier Macbooks using a thunderbolt to HDMI adaptor also work fine.

Suggestions?