In this paper, we address the issue of audio source separation with a single channel, i.e. the estimation of source signals from a single mixture of these signals. This problem is addressed with a specific configuration: source signals are assumed to be available before the mix is processed. We propose an original method that uses a watermarking technique to embed information about the source signals into the mix signal. Extracting this watermark enables an end-user who has no access to the original sources to separate these signals from their mixture. Thereby several instruments or voice signals can be segregated from a single piece of music to enable post-mixing processing such as volume control.