fixed static "sound" type for stage backgrounds and sprite duplicates

HISTORY.md

@@ -6,13 +6,14 @@
 * **Notable Changes:**
 * **Notable Fixes:**
     * JSON encoding for nested lists
-    * static "costume" type for stage backgrounds
+    * enforce static typing for stage backgrounds and sounds, and for sprite duplicates
 * **Documentation Updates:**
 * **Translation Updates:**
 
 ### 2022-02-07
 * lists: fixed JSON encoding for nested lists
-* objects: fixed static "costume" type for stage backgrounds
+* objects: fixed static "costume" type for stage backgrounds and sprite duplicates
+* objects: fixed static "sound" type for stage backgrounds and sprite duplicates
 
 ### 2022-02-04
 * new dev version

src/objects.js

@@ -1994,6 +1994,7 @@ SpriteMorph.prototype.fullCopy = function (forClone) {
             arr.push(snd);
         });
         c.sounds = new List(arr);
+        c.sounds.type = 'sound';
         arr = [];
     }
     c.nestingScale = 1;
@@ -7938,6 +7939,7 @@ StageMorph.prototype.init = function (globals) {
     this.costumes.type = 'costume';
     this.costume = null;
     this.sounds = new List();
+    this.sounds.type = 'sound';
     this.version = Date.now(); // for observers
     this.isFastTracked = false;
     this.enableCustomHatBlocks = true;