When I cast a video to a chromecast speaker device, it tries to send everything, . And I end up with a black screen and the audio coming out of the speaker device.
How can I cast just the audio, leaving video on my browser?
I don’t think you can do that from within Plex. You’ll probably need to handle that within the OS, somehow making the PC consider your Chromecast speakers a “regular speaker” connected to the PC.
… or given you’re using Plex Web: Chrome might have this covered for you → Using Chromecast Audio with Chrome - Chromecast Help
It has worked in the past. And then the popup started occuring where it tries to send all of it via the chromecast device.
