That's interesting and something worth looking at!
I did a bit of investigation into this the other day and I started to wonder if there were two things going on. Had the same report of some machines that wouldn't log in, I connected to one on SSH and checked top expecting to see the audiomxd at 100% and it wasn't. I then connected with remote desktop to see what was going on on the screen and immediately the audiomxd spiked.
I then repeated this whole process, rebooted the machine, checked processes, no audiomxd, connected ARD, saw audiomxd spike.
That made me wonder if the audiomxd thing is being caused by the remote desktop and we're tending to see it as we're remoting on to the machines to check them, so a red herring.
I then ran a trace on the audiomxd process to try and see if there were any clues as to what it was actually doing. They're hard to read but I did see bluetooth mentioned so I tried turning that off with blueutil and then once it was off I couldn't make it spike again.
So potentially connecting to a machine with ARD which has bluetooth enabled might trigger the audiomxd issue, but it's such an odd problem I'd be hesitant to say that's the one and only cause of it.
Perhaps this auth.db issue is the root cause of the stuck logins though I will see if I can test that here.
Ian