Stereo pairing for all apps is possible in macOS
- Open Applications > Music. This will be the Proxy Application.
- Select your paired HomePods from the AirPlay menu.
- Open Applications > Utilities > Audio MIDI Setup
You should see a list of Audio Devices including AirPlay.
HomePods are not listed in this window because they are managed by AirPlay.
The AirPlay device is only in the list when AirPlay is running.
4. Hit the + button then select Create Aggregate Device.
5. On the right side of the window will be a list of all the audio devices. Click the Use checkbox for AirPlay.
6. Use the volume icon on the top menu to select Aggregate Device from the list of devices.
Your sound output for everything on your mac will now play through the HomePods as stereo as long as the AirPlay connection stays open.
Trouble Shooting
Disconnects: If your AirPlay keeps disconnecting use AppleTV instead of Music as the Proxy Application. Play any show to bring up the player then select your paired stereo HomePods and pause playback. Then continue from step 3.
Volume: If your volume is too low adjust it through the AirPlay proxy app and don't forget you can control the volume of the HomePod through the + & - buttons on the top of the device.
Sound/Video Synchronization: In my experience I can see no disconnect with video such as Netflix or Disney+. However if you try to get fancy and add Built-in Output and AirPlay in a Multi-Output Device you will see a strong lag in AirPlay. Keep it simple and just use AirPlay with an Aggregate Device and your synchronization should be fine.
How does this work?
When an AirPlay application is open and AirPlay is selected as the default output device a
temporary AirPlay Audio Device is created. This Audio Device is handled by the same mechanisms as your other output sources and can be seen in the Audio Devices of the Audio MIDI Setup application.
Because the AirPlay Audio Device is temporary it will no longer exist when the proxy AirPlay application is closed or disconnected from AirPlay. These instructions detail creating an Aggregate Device. When the AirPlay Audio Device is removed it will be removed from the Aggregate Device too. The next time AirPlay is created it will be with a new instance of AirPlay and the old AirPlay Audio Device will be invalid.
So, does that mean you have to reconnect the AirPlay Audio Device in Audio MIDI Setup often? Well, yes. You can keep the same Aggregate Device in your list of Audio Devices and you can just keep assigning the active AirPlay Audio Device to the same Aggregate. In practice using AppleTV keeps the configuration valid even when putting the system to sleep.
Pro tip: rename your Aggregate Device to "HomePod AirPlay"