That was really odd. On Safari in Mac OS 11.1 Big Sur, YouTube videos were playing in fast forward with no audio. Using Chrome, the video playback was stuck, but the audio playback was fine.
My simple fix was to restart my Mac. Googling this issue, it seems like fast forward playback of any videos on the Mac has been an on-again-off-again issue for years. The “most unfortunate” solution offered was hosted on YouTube where I couldn’t see the solution in 2 seconds of fast forwarded playback. But the offered solution is accurate – force quit the coreaudiod service using Activity Monitor.
Here’s the solution described in text and augmented with video.