Fixes console error when setting null values as volume for the quiz sound

src/app/quiz/quiz-manager/sound-manager/sound-manager.component.ts

@@ -176,7 +176,7 @@ export class SoundManagerComponent implements OnInit, OnDestroy {
     if (isPlatformBrowser(this.platformId)) {
       const audioElements = document.getElementsByTagName('audio');
       for (let i = 0; i < audioElements.length; i++) {
-        (<HTMLAudioElement>audioElements.item(i)).volume = (this.config.volumeConfig[target] || 60) / 100;
+        (<HTMLAudioElement>audioElements.item(i)).volume = (parseInt(String(this.config.volumeConfig[target]), 10) || 60) / 100;
       }
     }
   }

src/app/shared/audio-player/audio-player.component.ts

@@ -40,7 +40,7 @@ export class AudioPlayerComponent implements AfterViewInit, OnDestroy {
     this._original_volume = value;
     this._volume = this._original_volume;
     if (this.audioElement) {
-      this.audioElement.volume = parseInt(this._volume, 10) / 100;
+      this.audioElement.volume = (parseInt(this._volume, 10) || 0) / 100;
     }
   }
 
@@ -81,7 +81,7 @@ export class AudioPlayerComponent implements AfterViewInit, OnDestroy {
   set volume(value: string) {
     this._volume = value;
     this.volumeChange.emit(this.volume);
-    this.audioElement.volume = parseInt(this.volume, 10) / 100;
+    this.audioElement.volume = (parseInt(this._volume, 10) || 0) / 100;
   }
 
   private _isPlaying = false;
@@ -123,7 +123,7 @@ export class AudioPlayerComponent implements AfterViewInit, OnDestroy {
   }
 
   public ngAfterViewInit(): void {
-    this.audioElement.volume = parseInt(this._volume, 10) / 100;
+    this.audioElement.volume = (parseInt(this._volume, 10) || 0) / 100;
   }
 
   public ngOnDestroy(): void {