I just installed 2.3 on my MAC running Mojave and tried to record from USB input and I’m getting nothing on the input monitor. I was able to monitor/record on my wife’s MAC suing the same device. (On hers 10.13 OS and 2.1.? I think)
Have you given Audacity permission to use recording devices? (see: https://forum.audacityteam.org/t/no-signal-from-usb-turntable-mojave-solved/50763/6)