This is something I’ve been trying to do reliably for years. I can stream anything I want easily with VLC or even just Chrome itself but I can’t get subs to work. I was able to make it work for a long time using a Chrome app called “videostream” but it now no longer works correctly on my system. It’s a bit confusing to me but it kind of looks from what I have read that Plex can apparently handle this? Most references to the idea seem to be for later chromecast versions but mine’s a 1st gen I bought in 2014. Could I use Plex to stream local media with separate or embedded srt files to my chromecast ?