Testing HDMI splitter with Nintendo Switch Animal Crossing

I have a BlackMagic UltraStudio Recorder 3G. I wanted to see if I could record any HDMI signal through it to Screenflow. The issue I had was I couldn’t see what I was doing. So I thought to use a HDMI splitter. Takes one input and splits into two identical outputs. In doing so, I could monitor the output at the same time as the recording. However, I ran into a new complication; no audio. I used a basic audio (headphone) cord from the switch, plugged it into a headphone to USB-C adaptor and used that as a microphone to record. You can hear the results. It’s very quiet. Once I amplified I ended up with a lot of hiss. As expected. So I need to consider a next step for audio.
BlackMagic UltraStudio Recorder 3G www.blackmagic...
HDMI 1-into-2 splitter www.amazon.ca/...
Headphone to USB-C adapter www.amazon.ca/...
Screenflow www.telestream...