Switch the speaker's input to line-in.

        Args:
            source (SoCo): The speaker whose line-in should be played.
                Default is line-in from the speaker itself.

def switch_to_line_in(self, source=None):
        """ Switch the speaker's input to line-in.

        Args:
            source (SoCo): The speaker whose line-in should be played.
                Default is line-in from the speaker itself.
        """
        if source:
            uid = source.uid
        else:
            uid = self.uid

        self.avTransport.SetAVTransportURI([
            ('InstanceID', 0),
            ('CurrentURI', 'x-rincon-stream:{0}'.format(uid)),
            ('CurrentURIMetaData', '')
        ])