For a while, my simracing PC was stuck on Windows 11 23H2 because I used a Windows Mixed Reality headset. WMR was deprecated and forcibly removed from Windows 11 by Microsoft in 24H2, so my only option was to stay on 23H2.
At some point in 2023 or early 2024, Discord audio stopped capturing when I was game streaming - no other changes to my machine. Since then I've periodically tried to get it working trying every possible fix or workaround available, but nothing worked.
I was finally able to update to W11 26H2 this week - and as soon as I did, Discord audio capture in games or application streaming magically started working again!
Without any more info, I can only presume Discord devs made a breaking change in an older build which resulted in audio capture being irreparably broken in 23H2 for certain hardware combinations.
What's even stranger is that on another machine, which I have intentionally kept on Windows 10 23H2, Discord audio capture for game or application streaming continues to work absolutely fine!
I raised support tickets with Discord and investigated logs myself over time, but I never had any meaningful response from them. Logs were inconclusive and didn't offer much by way of useful info on which to base investigations.
Unfortunately, I can't offer a solution in this post for why Discord on one machine started to have this issue with audio capture - I was never able to solve it. What appears to have fixed it was a W11 feature upgrade. Perhaps 24H2 would have fixed it -- but as I remained on 23H2 for much longer than most people (due to VR headset requirements), I could never test that.
Not everyone who has stayed on an older build of W11 may want or be able to upgrade to 26H2, but if you've also got broken Discord streaming audio and you're still on the fence about moving to the latest Windows 11, it may be worth contemplating the upgrade.