This app live captions any output to your sound device locally https://github.com/abb128/LiveCaptions
Whether I mute my output device, or selectively mute a tab or app it still works fine.
There's a similar feature baked into Win11, not sure whether that is processed locally/privately though
After seeing this post, I tried streaming the newest episode of Silo and found it wasn't working in my addon (which uses a4kscrapers).
My mother tried streaming Slow Horses (using same addon) after I told her and she said it worked fine. I guess it had already been cached and was therefore unaffected since the file I streamed was only released today.
Anyway I luckily only had 8 days left of RD, so I subscribed to Premiumize and it only took a minute to reconfigure the addon to use that instead.