If a neighbor plays loud music, and another neighbor calls the police, what happens if they noisy neighbors refuse to answer the door when the police show up?
Then the police have to yell through the closed door.
That would depend on the cops, but they could break in on the grounds that they suspected the loud music was being used to cover up someone screaming.
If the cops find out that the neibors are purposely not opening the door, they'll open it themselves.
If not, then the cops will walk around the house and knock on every window to get someone's attention to open a door. If they don't open a door when they meet eyes, the cops will enter for refusing to cooperate.
If none of those happen (cops never see anyone or sense they're not being locked out) then they'll leave and come back at a later time unless there's currently loud noises coming from in the house. If that's the case then they'll get a warrant and go in.
Most of the time, unless it looks like a horrible emergency, the cops are required to get a warrant before busting through the door.