set PlaybackManager.DummySoundPlayed = true after dummy sound play (#38, #101)

this was already set in the sponsor audio player demo (wordpress), so it's tested and working on iOS.

src/Playback/PlaybackManager.ts

@@ -203,6 +203,8 @@ export class PlaybackManager implements IPlaybackParametersListener {
         } else {
             (source as any).noteOn(0); // this was the old way to start a sound
         }
+
+        this.DummySoundPlayed = true;
     }
 
     public playVoiceEntry(voiceEntry: VoiceEntry): void {