Is there a broadcast action for volume changes?

There is no broadcast action, but I did find you can hook up a content observer to get notified when the settings change, volume of streams being some of those settings. Register for the android.provider.Settings.System.CONTENT_URI to be notified of all settings changes:

mSettingsContentObserver = new SettingsContentObserver( new Handler() ); 
this.getApplicationContext().getContentResolver().registerContentObserver( 
    android.provider.Settings.System.CONTENT_URI, true, 
    mSettingsContentObserver );

The content observer might look something like this:

public class SettingsContentObserver extends ContentObserver {

   public SettingsContentObserver(Handler handler) {
      super(handler);
   } 

   @Override
   public boolean deliverSelfNotifications() {
      return super.deliverSelfNotifications(); 
   }

   @Override
   public void onChange(boolean selfChange) {
      super.onChange(selfChange);
      Log.v(LOG_TAG, "Settings change detected");
      updateStuff();
   }
}

And be sure to unregister the content observer at some point.