We propose a method for embedding a robust forensic tracking watermark within the AAC decoder during audio playback. For predefined frequency bands, the method intercepts and modifies the scale-factors, which are present in the AAC bit-stream. It thereby modulates the short-time envelope of the band-limited audio and embeds a watermark which is robust to various attacks such as recompression and acoustic transmission. Due to its low complexity-a few additions per AAC frame-the method is suitable for implementation even in resource constrained devices such as portable players. Furthermore, decode-time embedding enables dynamic payload assignments which may be used to signal playback time and location.