Thanks for the reply!
We've tried a lot of different things, so they may be running together in my mind slightly.
We have some older (from October, 2020) documentation with screenshots of how we had it set up. All of the inputs on the Audio tab were set to disabled. But, even when we enable them, and select which input we do want (which does show up there), there's never any activity in the VU meter, and if we make a test recording, it comes out silent.
The tricky part is that we can see in the Windows control panel the input jumping as we are sending sound through it. As a further test, we installed Audacity, and it was able to record from that line in with no problems.
We're this close to just uninstalling OBS and reinstalling it again, but we're not sure we'd keep our settings (or if we would want to, maybe that's where the issue is?), or if that would even fix the issue. From the logs, there were some errors, but we're not really sure what a 'normal' state would look like. Does it matter if an encoder is not loaded or installed? Can we install one manually?
10:31:42.017: [CoreAudio encoder]: CoreAudio AAC encoder not installed on the system or couldn't be loaded
10:31:42.017: Failed to load 'en-US' text for module: 'decklink-captions.dll'
10:31:42.018: Failed to load 'en-US' text for module: 'decklink-ouput-ui.dll'
10:31:42.039: [AMF] Unable to load 'amfrt64.dll', error code 126.
10:31:42.041: [AMF] AMF Test failed due to one or more errors.
10:31:42.041: Failed to initialize module 'enc-amf.dll'
10:31:42.059: [obs-browser]: Version 2.9.1