Methods and apparatus for determining the existence of an external clock over a digital input port on a computer. In one embodiment, the external clock is validated, and a lock is performed when the clock is valid. Whenever a loss of the lock is detected, and, if a re-lock is likely, the apparatus is muted so that audio artifacts that would otherwise be heard are minimized. The methods and apparatus also provide automatic re-locking to the external clock when a sampling rate change is detected.