In recent years research in the area of artificial bandwidth extension (BWE) of speech signals has made significant progress. The intention of BWE is to produce wideband speech with a cut-off frequency of for instance 7 kHz from a narrowband version (e.g., with telephone bandwidth, i.e., 300Hz - 3.4 kHz). The respective algorithms are based on the estimation of parameters of a source model for speech production given the knowledge of the narrowband signal. A theoretical performance bound on this estimation has been formulated in [1]. In order to overcome this boundary we propose the transmission of (compact) side information which can support the parameter estimation. Since a common additional channel would conflict with the requirement of backwards compatibility in narrowband communication systems, the side information is embedded as digital watermark into the narrowband speech.