intro = new Audio.newMusic(Gdx.files.internal("core/assets/OpenSong.mp3"));
In the javadocs this is how it says to do it
https://libgdx.badlogicgames.com/ci/nightlies/docs/api/com/badlogic/gdx/audio/Music.html
Here: https://www.gamefromscratch.com/post/2013/11/19/LibGDX-Tutorial-8-Audio.aspx they used a lowercase audio instead of uppercase but when changing it to lower case that didn't work either.
It's as if audio/Audio just don't have that method.
Your code should import the class com.badlogic.gdx.Gdx and then you can use
intro = Gfx.audio.newMusic(Gdx.files.internal("core/assets/OpenSong.mp3"));
Alternatively you can write
// where all the import statements live
import static com.badlogic.gdx.Gdx.audio;
// where you try to load the music
intro = audio.newMusic(Gdx.files.internal("core/assets/OpenSong.mp3"));
Your code tries to create a new Audio instance (this will not work, since Audio is just an interface - you would need to create an implementation for it).
Just replacing new Audio with audio will also not work (unless you statically import com.badlogic.gdx.Gdx.audio, as in the second example) - audio alone would mean a field or local variable audio.
