Are you using "Default" for any of your audio connections? That's a common source of sudden problems when you haven't changed anything.
In fact, something did actually change, but you didn't do it. The operating system switched *its* selection to a different device - maybe legitimately, maybe not, but it did - and OBS's "Default" follows that.
No matter how it ends up that way, if you Monitor a source that picks up the same output device that the Monitor is sent to, that's a feedback loop, and it does what you describe.
So:
- Always choose a specific device in OBS, or "Disabled" if you're not using one that you can't otherwise get rid of. Never use "Default" unless you explicitly *want* that auto-switch for a specific reason, and you're okay with the other effects of that auto-switch.
- Always keep an eye out for feedback loops. Know how signals run around your rig, and get rid of any loops that might crop up.
- It may help to study an old audio mixing console that has pre-/post- and other routing switches on it. It's not just about grabbing "a signal", like a specific mic or the output of the computer speakers. It's about *where* in the chain of processing you grab that signal. Different tap points give you different results from the same name.
- In a typical OBS rig, you don't get to choose the tap point. It just is where the operating system or audio driver has it. But knowing the concept goes a long way in understanding what it means, to say that the output capture tap point is post-*everything*.