Using RCA input while PC plugged into HDMI

I'm trying to use a USB DAC for audio in ROON. I have a Mac Mini plugged into HDMI, but I'm able to direct audio to USB. The DAC has analog output via RCA. I can get audio from HDMI, but when I switch to the USB DAC no audio is heard even though the DAC seems to be working. Is there a way to assign audio input to RCA?