The Audio Routing Frustration
macOS is designed to be as “plug and play” as possible. When you connect an external monitor via HDMI, DisplayPort, or USB-C, the operating system detects the new hardware and automatically assumes you want to use the monitor’s built-in speakers for your system audio. It instantly switches your audio output away from your high-quality external speakers or headphones to the (usually terrible) monitor speakers. If you frequently dock and undock your MacBook throughout the day, you have to constantly dive into the Control Centre to manually switch your audio routing back to your preferred device.
Locking the Output Device
Unfortunately, macOS does not provide a simple toggle switch to say “never use monitor speakers.” However, you can use a free, lightweight, third-party utility called “Audio Hijack” or a simpler open-source script tool like “SwitchAudioSource” to force the Mac to respect your primary audio choice.
If you prefer a built-in workaround without installing software, you must use the Audio MIDI Setup utility to create a stubborn “Multi-Output Device.” Open Spotlight (Cmd+Space) and search for Audio MIDI Setup. Click the small “+” button in the bottom left corner and select Create Multi-Output Device. In the list of available devices on the right, check the box for your preferred speakers or headphones, and ensure your external monitor is completely unchecked. Now, right-click this new “Multi-Output Device” in the left sidebar and select Use This Device For Sound Output. Because this is a custom, aggregated virtual device, macOS will prioritize it. When you plug in your monitor, the operating system will see that the virtual device is active and will refrain from automatically hijacking your audio stream.