How do I get AirPlay to work?

Important: Bug in macOS Sonoma

Due to a bug in macOS Sonoma AirPlay isn't working if your Mac or AirPlay device are connected to your network via Ethernet. To work around this bug both your Mac and AirPlay device must using WiFi.

1. Disconnect your Mac from ethernet and use WiFi.
2. Make sure your AirPlay device is connected to WiFi.

Source:
https://www.reddit.com/r/MacOS/comments/16thuge/homepod_not_playing_via_airplay_on_sonoma/

How to use AirPlay

Sound Control only works with AirPlay if you have the System Extension installed. For more information about the System Extension see: Installing PlugIn on macOS 11.4 or Later

Once you have the System Extension installed, Sound Control will work with AirPlay if you follow these steps:

1. Open Audio MIDI Setup

2. Click on the + button in the lower left corner

3. Select "Connect AirPlay Device" and select the AirPlay device that you want to use.

After selecting the AirPlay device your Mac will select the AirPlay device as your Mac's System Output.

Now Sound Control should be able to apply volume controls and effects to the audio that is played to AirPlay.