Anime where people live in a virtual reality world to escape from reality

Question

I'm searching for an anime about a virtual reality world in which people "live" in there, because they want to escape from reality. In this world they can become the person they want to be, but the main character and some others want to get back to the reality, but a group of people who really wants to stay in this world won't let them get out of the world.

Also the main character has created an AI which out of her own will wants to give him happiness and so decided to create the vr world. In this world she is a singer and she turns others into monsters if she sings. I also remember that in the first scene the MC is in a metro going to school where later the AI appears and turned students into monsters killing other students who didn't turn into monsters and also when killed in the VR world you die also in the real world.

Answer 1

Caligula (2018). I just spent 30 minutes looking for this and created an account to post it.

From MyAnimeList:

What is happiness? Ever the fan of psychology, questions such as this are ones that high school student Ritsu Shikishima likes to ponder as he spends his peaceful days with his friends. His perfect world, however, begins to unravel when he hears a strange voice obscured by static, pleading for help. This voice belongs to μ, a beloved pop idol, whose singing begins to have an adverse effect on the world. Before Ritsu's very eyes, the faces of his friends and family become distorted by glitches as the sound of μ's voice transforms them into Digiheads: berserk monsters bent on the extermination of all those who begin to awaken to the true nature of their existence.

Realizing that he is trapped in a virtual world created by μ called Mobius, Ritsu must now gather everyone else who has managed to realize the truth before they are all eliminated. Together, they will use their newfound powers and weapons granted by their emotions—known as the Catharsis Effect—to fight off the mysterious group known as The Ostinato Musicians as they struggle to escape.