Here’s a rather insidious problem I encountered this weekend. YouTube wouldn’t play videos – it wouldn’t play ads, it would just spin like below:
The strange thing is that it happened on my Mac Mini (2018) with Big Sur with Twitch and YouTube, it happened on every browser, it didn’t happen on my other Macs, and it didn’t happen on any of my other devices. I actually spent over two hours with Apple Support as they walked through everything they could think of, from resetting SMC and NVRAM and browser settings. The problem didn’t go away until we did a refresh reinstall of Big Sur.
But then this issue happened on my old Catalina iMac (early 2013) as I was setting up audio. I was suspicious that it wasn’t an installation issue and that it was some kind of setup bug.
It might be your audio Multi-Output Device
If you do pro audio on your Mac, or if you do streaming, you might have created a “Multi-Output Device” to use multiple audio interfaces simultaneously. Turns out that the order in which you enable these matters and that you need to enable them in the right order. I can’t tell you right now what the right order is, but I can tell you how you can diagnose this.
- Before you start, try selecting the Built-In Output as the sound output. If the video plays, then it’s the Multi-Output Device. If it still doesn’t play, it may be some other problem and I don’t think these instructions will help.
- Next, in Preferences, select the Multi-Output Device as your Sound output.
- Go to the Audio MIDI Setup program.
- Go to your Multi-Output Device.
- Start a video. Anything on YouTube is fine. It should begin playing, likely using your built-in Mac speakers.,
- In the Multi-Output Device, enable the devices, one at a time. In general, I think the order that works is to first enable the Built-In Output, then your hardware audio interfaces, then internal software mixers like BlackHole/Soundflower or ZoomAudioDevice.
- If the video stops, then disable the device, and enable a different one.
- Stop when you’ve enabled all of the devices without the video stopping.
So if videos aren’t playing in Safari, or any other device, and you’ve enabled Multi-Output Device, then check this setting. Hopefully this will solve issues with online videos not playing on your computer!
Irwin Kwan 
Hi Irwin, I’m having a similar problem as yours. Did you identify how to fix this problem?
I’m using two bluetooth headphone configured with Audio MIDI Setup and most of the time I have to kill coreAudiod process on Activity Monitor and refresh the browser to resume YouTube video. And how I’m not using Built-in Output, I don’t know how to set the order to works.
I turned all the internet searching someone with the same problem as mine but I couldn’t find. You are the one that has closest situation as mine.
It’s been a while since I’ve encountered this issue since I’ve updated my Macs to M1. I don’t have any solutions for you off-hand, sorry. Maybe I’ll try at home later and see if I see this.