Sound creature

The Sound Creature was an entity inhabiting an alternate universe, which may have been brought into it, accidentally, by the 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.

History
The Sound Creature was encountered when the Doctor and Charley Pollard were stranded in an alternate universe. It was able to echo simple sounds, evolving the capacity for communication during the course of the experiment it had become a part of, after having out-evolved the species that would've arisen originally, causing them to die out. It 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, although they managed to evolve themselves to keep up with it. Desperate, it attempted to convince them to die for it's sake, but they both refused, allowing them to out-evolve it and become the dominant life form in the experiment. With this new power, the Doctor silenced it forever, choosing not to kill it, and left it in the experiment. (BFA: Scherzo)