I should add then when I use embed code such as this...
"data:text/html,<iframe id=\"ytplayer\" type=\"text/html\" width=\"640\" height=\"390\" src=\"http://www.youtube.com/embed/" & Media::YouTube & "?autoplay=1\" frameborder=\"0\"/>"
the audio plays, but there's no video.
I feel like an idiot. The problem only showed up when I had FileMaker's window scaled down.
The screen is designed to be 1920x1080, and my MacBook Pro's screen is too small to see everything. When I went to that record now at 100%, it works fine.