An audio device and method for adding a watermark data to digital signals which represent audio generates a digital signature by applying a hash calculation on the hardware address of the audio device. The audio device searches for an orthogonal code in a walsh matrix. The audio device decodes audio digital signals according to the orthogonal code to output decoded data. The audio device loads the orthogonal code into an identification information of the audio device to form the watermark data in accordance with a determination that the decoded value is not equal to one of a plurality of specific values, wherein the identification information includes the hardware address of the audio device. The audio device adds the watermark data into audio digital signals.