I am having this exact problem. I am using 8.5.30 through a UR22 into an iMac with OS Mojave 10.14.2. I have reinstalled, updated etc, I have been using Cubase for about 12 years so I don’t think that inexperience is the cause of my problem. I have checked that I am using the right audio inputs, busses etc numerous times, armed the track, monitoring etc but no audio is being recognised by the DAW even though it is coming through the Mac system and I can hear it fine. It was working last time I used it and now for some reason it is not. I also have a Mac Pro laptop running OS Mojave and Cubase 8.5.30 is working perfectly. Any solutions or help would be greatly appreciated.