When you're using the Music App, you can choose the "AirPlay icon" -  or you can select the system-wide audio settings, then select sound and tap the AirPlay icon. We recommend you to choose the AirPlay from your Music App that lets you use the latest AirPlay 2 protocol. While starting from the system-wide audio settings, it'll output every sound on your system to WiiM Mini (including your notification sound) and uses legacy AirPlay protocol.