made old sounds removed on adding new sound

public/js/cell.js

@@ -106,8 +106,12 @@ export default class Cell {
 		this.keyButton.html(" ");
 	}
 
-	//TODO this will probably do more stuff later
 	assign(sound) {
+		//if there was an existing sound, remove it
+		if (this.sound !== null) {
+			this.sound.unload();
+		}
+
 		this.sound = sound;
 		//we need to check this, in case the mode was switched to loop before a sound was loaded
 		this.sound.loop(this.mode === modes.LOOP);