Form1 frm1 = new Form1();
This creates a new instance of your form, it doesn't reference the form playing the music.
You need to pass a reference to Form1 to Form2. One tactic could be to add a property to Form2 called MusicForm or something similar - then when you create form2 you can pass a reference to form1.
public partial class Form2 : Form
{
public Form1 MusicForm { get; set;}
...
...
Then you can modify Form1:
public partial class Form1 : Form
{
Form2 frm2 = new Form2();
frm2.MusicForm = this;
public WMPLib.WindowsMediaPlayer wplayer = new WMPLib.WindowsMediaPlayer();
Then call:
this.MusicForm.wplayer.controls.stop();
Of course, you don't need to pass the whole form if all if all you want to pass in the WindowsMediaPlayer instead - you could also just pass that in as a property.