We propose a novel method for embedding robust forensic tracking watermarks to complement encryption in electronic music distribution applications. The watermark is embedded at the player during the AAC decoding process through modification of scale-factors in pre-defined frequency bands. It thereby modulates the short-time envelope of the decoded audio in the corresponding band. The resulting watermark is robust to various attacks such as re-compression and acoustic transmission. The method, due to its negligible computational overhead, can be used in resource constrained devices such as portable players.