An MP3 player was a device used to play music. Luke Tillyard played Faithless on his MP3 player to cause C'rizz to fall unconscious with the noise. (PROSE: Salva Mea)
In 1974, the Eighth Doctor used Lucie Miller's MP3 player, mispronounced "Empty 3 player" by a young Aunty Pat, to invert the sound waves, used by the Only Ones to pit-stop on planet Earth and consume humans. These sound waves turned into troughs, which trapped the Only Ones in the MP3 player. The Doctor put them forever on shuffle mode. (AUDIO: Horror of Glam Rock)