Why would it do this? I have gotten reports of this over and over, and looking back on VOD's.....yes. I have a 2-PC setup and use an Elgato device (use it for console capturing as well). There are no problems whatsoever with video, and everything always sounds fine in my headphones, so it's gotta be the capture that's the issue. Does anyone have any ideas?

This Elgato device, if it helps/matters -> https://www.amazon.com/Elgato-Game-Capture-PlayStation-Definition/dp/B00840353W

Have you tried local recording and seeing if the audio fluctuates? That would help narrow down whether it’s an actual capture problem or an issue when streaming.

I don't think the problem is streaming, I think it's PC capture. This issue does not occur when I stream console games. And there's actually a (small) chance I may have figured this out, if it's still an issue I'll update.

So what I thought might have worked didn't. But I was able to isolate the problem. The problem is the manner in which I had setup to send sound to both the Elgato device and my headphones simultaneously. I used the method detailed here -> https://windowsreport.com/play-sound-2-devices-windows-10/

When I send sound only to to the Elgato device, everything is 100% fine.

So is there a better method to send sound to both?

I see a few things you could do. Either instead of listening to the game pc, listen to it via the stream pc. Either via OBS or voicemeeter. Or use something like virtual audio cable to "duplicate" the signal.

Stream PC has about a 10 second delay, so that's not really feasible. What I ended up figuring out as a workaround was to basically just treat the gaming PC completely like a console. Set my TV to the input listening for the capture device and plug my headphones into the TV. The capture device already splits audio, so it works okay. There is just the slightest, practically imperceptible, delay. But I can 100% live with it, even during speedruns.