YouTube videos play back at unwatchably fast rate with Safari on the latest Big Sur beta... anyone else?
This is a commonly reported bug that occurs 'sometimes'. Another solution, short of rebooting, is to kill the coreaudiod process.
You can do this in Activity Monitor, or in the Terminal with sudo killall coreaudiod
https://9to5mac.com/2019/07/29/macos-fast-video-playback-bug-fix-coreaudiod-video/