A mobile device includes a processing module, an external connection interface and a current monitoring unit. The external connection interface is electrically connected to the processing module and includes a first pin, a second pin and a third pin. The first pin is configured to transmit a first speaker signal and the second pin is configured to transmit a second speaker signal. The current monitoring unit is electrically connected to the processing module and the third pin of the external connection interface and configured to monitor a current state of the third pin and feedback the monitored current state to the processing module. The processing module is configured to provide one of a plurality of different voltage levels to the third pin according to the monitored current state. A noise-canceling earphone is also disclosed.