Elgato Game Capture HD60 S features an analog 3.5 mm stereo input. Only one input, HDMI or analog can be used at a time.
To change the audio input from HDMI to analog:
1) Open OBS Link from the macOS status bar
2) Click on Preferences… to open the Preferences window
3) In the Preferences window, from the Audio Input dropdown, select Analog Line-in
Now audio from the analog line-in will be used instead of HDMI audio.
Use the Audio Gain slider just below the Audio Input selection to adjust the analog audio input volume.
To capture HDMI audio again, go back into the preferences and select HDMI from the Audio Input dropdown menu.