Methods of processing audio to convey auxiliary information therein without audible evidence of data alteration. Various arrangements are disclosed. One receives plural bit auxiliary data in which the auxiliary information is included, and modulates a carrier signal with the auxiliary data to yield intermediate data. The intermediate data is then summed with the original audio to produce the encoded audio. The carrier signal can be tailored to enhance performance, and the intermediate data can be scaled in amplitude so as to better hide the auxiliary data within the audio. One application of the technology is in monitoring, where the auxiliary information can be used to track usage of audio or audio-visual content.