Revert "Only load audio src in ListenBox if it's defined (#442)"

This reverts commit d31de940.

web/src/lib/components/listen-box.tsx

@@ -167,9 +167,7 @@ export default class ListenBox extends Component<Props, State> {
         </div>
         <audio
           className="audio-box"
-          // Only include the src attribute if the source is defined
-          // (empty src attributes are invalid)
-          {...this.props.src && { src: this.props.src }}
+          src={this.props.src}
           preload="auto"
           onLoadStart={this.onLoadStart}
           onCanPlayThrough={this.onCanPlayThrough}