Thanks, I didn't see the FAQ. I copied the 32-bit DLL files as recommended. The YouTube error message disappeared, but now the video just buffers indefinitely and never plays... I can't see any related error in the console.
I am running through VirtualBox on an AMD 3600, so SSE/ASM builds should work.
EDIT: Seems like the new DLLs weren't properly loaded until I rebooted; restarting the browser wasn't enough. All good now, thanks.