AirPlay audio works, but not video.

Content that is not supported for AirPlay (by the content provider), may still send the audio through. Could that be happening? Can you tell us what you were trying to AirPlay?

If AirPlay or screen mirroring isn’t working on your device - Apple Support

That video link AirPlays fine here from an iPad in Safari, as well on from a MacBook Pro in Chrome and Safari, with video and audio. I can’t replicate your issue.

These specific broadcasts could and can also be viewed on the YouTube app on Apple TV, in the C-Span Channel. https://www.youtube.com/cspan

MacBook Air Early 2015, Mac OS 10.11.6

On a side note, are you aware that this 2015 model can be upgraded to 10.12/10.13/10.14/10.15/11.x ?

I don’t think it matters for AirPlay, though.