I am also having trouble with the latest mpv, which required an update to libplacebo and ffmpeg on my Archi Linux system.
Nothing plays. Sending a stream results in an emms error of nothing found. Sending a file like an mp3 results in an odd behavior where cpu usage goes up and in the mode line 00:01/03:45 (for instance, for a 3:45 length track), keeps blinking at 00:01 but still no output, although the mpv process is active.
I communicate with sockets.
Reverting back to the previous versions of ffmpeg, libplacebo, and mpv resolve the problem (no surprise there).
This is surely not an emms bug per se, but somehow the way emms needs to interact with mpv may have changed. I haven't as yet been able to trace this down any further. Of course, there could be a new problem strictly in mpv which emms can't fix. I don't know at this time.