You would typically see this behaviour if you tried to boot a 64-bit OS requiring a 64-bit bootloader but the device's bootloader was set as 32-bit in the BIOS. So first check what is set in the BIOS by booting and pressing F2 and then boot from a USB which has been written with the appropriate OS. Finally if you are trying to boot a Linux OS bear in mind that the kernel needs to be at least v4.12 for audio over HDMI to work.
Thanks...your comments did help. The boot options given in the BIOS are Win-32, Win64 or Android. I had exercised all 3 of these options a few weeks ago to no avail, but this was before we ordered the 'approved by Intel' USB stick. With the new stick, I was testing it with the last setting I had tried, which was Win32. Setting it to Win64, the new stick does boot. Strange though, as the UEFI image is Linux, not Windows.