Development of a watermarking tool for MPEG-1 audio files using Layer I, II, and III encoding. The application is robust, fast and secure. It is capable of marking each audio file several times and later recovering the watermark even from fragmented files. The watermark is undetectable for humans, yet it is robust enough to withstand file conversion and audio modifications. The tool is designed to be used on-line, which means it can mark files in a very short timespan.