Apple released macOS High Sierra 10.13.6 on Monday, adding support for AirPlay 2. The update lets you play music through paired HomePods as well as multiple AirPlay 2-capable devices via iTunes.
The update also fixes an issue that prevented the Photos app from recognizing AVCHD media from some cameras, along with a bug where Gmail messages couldn’t be moved to other accounts in the Mail app.
macOS High Sierra 10.13.6 is a free update. You can find it by going to Apple menu > App Store, then click the Updates tab.