I have the same problem and found this answer on Reddit.
Now looking for where to change settings to stereo.
"The initial 'conversation' between the app and the Chromecast is basically "hey Chromecast, what do you support for audio?" , the Chromecast responds with "I support 5.1!", and the app responds "oh sweet, I've got 5.1 audio, gonna send that over right now!".
So because of that, the app is sending a 5.1 channel audio output to the Chromecast. Problem is, your actual TV doesn't support 5.1 audio, and is deciding to only play the Front Left and Front Right channels, which are typically just background noise and sound effects. Pretty much all speech is done through the Center channel, which is missing.
Some apps are better than others about this. Netflix, for instance, lets you just pick a different audio track (labelled Stereo) with a press of a button. Others, like Disney+ and HBO, lack that feature completely. You are stuck with whatever settings it decided on during that initial connection.
The reason you may not have gotten issues with it previously, is because a lot of Disney+ content only comes in Stereo sound to begin with. I can only think of a handful of things I've tried to watch on there that actually caused issues (Avatar, and Tron: Legacy I believe).
As for how to fix it.......
These are known issues with these apps. Issues that the dev teams don't have any priority on fixing. Send complaints to their support teams, and eventually they might pay attention to it.
Some TVs have an audio output, so that you could connect to a surround sound system. Some of those TVs has the option to disable that feature in the settings (usually labelled "digital audio passthrough" or something). Sometimes, that will end up telling the Chromecast to force Stereo mode. I don't know much more about that, since it doesn't work for me."
