A manifestation of Haruhi Suzumiya's antimatter-dispersing oscillations perceived as sound waves (called sound worm by Kyon) was created by Haruhi Suzumiya's powers and located in phase transposition space much like the cave cricket data organism. The space appeared as a concert hall, rather than the physical location where the space was entered.
The creature forced anyone who listened to First Good-Bye to painfully hear it in their head, even Yuki Nagato (though the effect on her was less due to a "difference in physical constitution"). Its body, over 200 meters long, was composed of a musical score: five lines with scattered notes. The lead treble clef acted like fangs. The creature could attack with swarms of "notes" and could alter voices. It could not be damaged by physical attacks, only by the manipulation of sound waves.
The sound worm appeared in the audio drama Sound Around.
Haruhi wanted to create a band centered around the SOS Brigade, who could create a demo tape, which would allow them to play at a live music festival. Haruhi created a song called First Good-Bye. However, the Brigade failed to add more members to the band and weren't talented at singing.
Haruhi subconsciously created the "sound worm", which bombarded the minds of anyone who had heard the song and forced them to hear it endlessly. The Brigade awoke one morning all hearing the song (except Haruhi). If the demo tape was heard, or worse the song played live in front of a large audience, many people could be affected.