Why is the coreaudiod daemon using 5% or more CPU?
The high CPU usage of coreaudiod is due to the ambient noise reduction software working (surprisingly, even when the built-in microphone is not in use).

Go to System Preferences > Sound > Input and uncheck the Use ambient noise reduction box.

Then, either reboot or run this one-liner to reload coreaudiod, and you should see coreaudiod CPU usage drop to 1% or less.