Unity 4.6 Checkbox! How does it work?

You can write a function that will be called on value changed for the toggle. Just write this function and add under the Toggle (Script) component there is an option for On Value Changed. Click the plus button and then drag and drop the game object with the script containing for this function. Then select the function from the list. (This is the same as On Click for a button)

Remember that this function have to be public and has a return type of void.

You can check the state of toggle using isOn variable of that toggle.

The script will be:

 using UnityEngine.UI;
 
 public class ChangeVolume : Monobehaviour {
 
 [SerializeField]
 Toggle musicToggle; //Drag and drop your toggle game object here or you can even get a reference to this in Start()
 
 public void SetMusic()
 {
     if(musicToggle.isOn) {
         // Set music On
     } else {
          // Set music Off
     }
 }
 }