The sound creature was an entity inhabiting the Divergent Universe. It may have been brought there accidentally by the Eighth Doctor and Charley Pollard. It manifested itself as sound in the form of music and fed on sound, particularly emotional sound, or sound coming from the vocal chords.
The sound creature was encountered when the Eighth Doctor and Charley Pollard were stranded in an alternate universe. At first, it was only able to echo simple sounds. It evolved the capacity for communication during the course of the experiment it had become a part of. It had already out-evolved the species that would have arisen originally and which had died out. The sound creature eventually learned to make music and master speech, and almost out-evolved the Doctor and Charley when the Doctor attempted to feed it enough to get it to evolve the ability to reason. The Doctor and Charley managed to evolve themselves to keep up with it. Desperate, it attempted to convince them to die for its sake, but they both refused. This let them out-evolve it and become the dominant life form in the experiment. With this new power, the Doctor silenced it forever, chose not to kill it, and left it in the experiment. (AUDIO: Scherzo)