To AirPlay from Mac to HomePod, your Mac and your HomePod need to be on the same Wi-Fi network.
Make Sure Volume is Enabled in Menu Bar
It's easiest to AirPlay sound to the HomePod from a Mac using the menu bar on your Mac, but to do that, you need to make sure your audio settings are readily accessible. By default, audio settings aren't available on the menu bar, so you'll need to fix that.
- Open up System Preferences.
- Choose Sound.
- Select the "Output" tab.
- Check the "Show volume in menu bar" box.
Selecting HomePod as an Audio Output Device from the Menu Bar
Once sound controls are accessible from the menu bar, it's simple to route the sound from your Mac to the HomePod.
- Click on the volume button in the menu bar.
- Under the "Output Device" list, choose the HomePod, which is labeled as the room that it's in. Mine is Office 2, because I have two AirPlay devices in my office.
When using the HomePod this way, as an external speaker for the Mac, there's no option to use Siri to do things like skip songs. tYou'll need to control playback on your Mac, but you can use the physical controls on the HomePod to adjust volume.
AirPlaying from iTunes
For the most part, you need to use the audio output settings to AirPlay music from a third-party service to the Mac, but if you're playing content from iTunes, you can use the built-in iTunes AirPlay controls.
- Open iTunes.
- Play a song from Apple Music or your music library.
- In iTunes, click on the icon.
- Select the HomePod icon.