Let us start over, again. My assumption is the problem is NOT the Codex DAC (gut feeling). Any chance of borrowing another DAC from a friend and see what happens? This test will confirm the CODEX DAC works or does not work.
Let us also assume, for minute, the issue is either the Mini or ROON. What do you mean by inaudible audio? Does that mean NO sound or just a little sound? I had a similar issue when streaming from my iPad and had to turn the iPad volume control up to its highest setting. A little sound means a volume control setting, somewhere, needs to be increased. In other words, if the CODEX DAC is not working, you will get no sound.
Does your Mini, Roon, or anything else have a volume control that needs to increased? Please double check again (sorry), all setting on these devices to ensure their volume setting is set high. If none of the above, what else could it be?
If nothing, please call Ayre Acoustics for advice. Please keep posted.