Well that's the thing, all of the Sound, video and game controllers drivers are the same (and the latest) so it must be something else. But what? For example, the system that works doesn't have CloneDriver installed but the other system does (did) so I uninstalled it but it didn't have any effect. I swap drives and it works with one but not the other - so I assume it's not BIOS related.
The installed drivers don't seem to be the issue so maybe something is, or isn't, installed/the same between the two systems - which there are probably lots since one system was newly created and will not have, for example, some older Windows updates that the older system will have.
BTW, I cloned the platter drive (which has the functioning audio) to another SSD and although I haven't tested it much, it seems to have been successful. So I do have a functioning system but I would still like to try and understand what's going on with the system with the non-functioning audio.