I haven't used this in a while (went to Sonos) but I used it for quite some time with no problems.
https://rogueamoeba.com/airfoil/
you can pick to stream from a single app or system audio.
Single app is nice, because you don't get notification noises out of your remote speakers.
you can also choose a different browser than normal for youtube, (chrome if you normally use safari or vice versa)
that way, if you go to a site in safari with auto play video, it doesn't come out of your remote speakers,
it can also send to google cast devices, as well as their app on iOS or android devices.
and small disclaimer.
there is a free trial (it will only stream for 10 minutes at a time)
so be sure to try it before you buy it. (you get an activation code that removes the streaming limit)
(only because someone here got pissed at me when they purchased it right off the bat, and it didn't do what the needed)