I just finished building my Ramiel cosplay for OZ Comic-con the character is from Neon Genesis Evangelion and is also known as the 5th Angel from the original series. I took it to my Anime club at Deakin Uni to have a test run. The sound is Ramiel's song coming from inside the outfit.

I got pictures uploaded at cosplay.com as well.
http://www.cosplay.com/costume/428279/

People have asked for the sound effects so here are the links:
Ramiel First Main Canon Blast + Explosion.mp3 (318 KB)
https://mega.co.nz/#!xsZhya7C!YlrmXihhEmT5vwzlLLQpCWVn4oZkUQ8ZojQypFjUZ5E
Ramiel Death Scream.mp3 (100 KB)
https://mega.co.nz/#!d8BGWTAJ!R3JxI2gqbd3ERVfug-gX2TdYe5A48MSK33sp-cDFZLM
Ramiel Singing 5 Minutes .mp3 (4.6 MB)
https://mega.co.nz/#!1tBTmZzK!YQzveHfEukEtyuV31MxZjy4lshVpthd7Tw-YoZ7EMro
Ramiel First Main Canon Blast.mp3 (71 KB)
https://mega.co.nz/#!khJxzbgY!Du9aPVqNxU1kLeVvU8Wvh0lmmQ96twS4Yhc3hijloFk