Sorayume no Uta

In the future, a devastating war has forced mankind to take refuge under ground. While most of humanity is content, several people dream of the impossible–of seeing the sky. The Narise Corporation has been researching just how to do that, using a special robot, Ciel, who has the unique ability to conjure up images of the world above ground through his songs. However, when murder strikes the company, Ciel has to run away...but he won’t ever forget his promise to lead his friend to the sky.
(Source: Storm in Heaven)