Revert "Revert "Only load audio src in ListenBox if it's defined (#442)" (#459)"

This reverts commit fe8f6433.

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

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