code: @Override
      public final Sample getVocalization(char num) {
          try {
              Integer.parseInt(num + "");
          } catch (NumberFormatException e) {
              throw new IllegalArgumentException(
                      "Expected <num> to be a number, got '" + num + "' instead.",
                      e);
          }
  
          int idx = Integer.parseInt(num + "");
          String[] files = _voices.get(idx);
          String filename = files[RAND.nextInt(files.length)];
  
          return SampleUtil.readSample(filename);
      }
docs: {@inheritDoc}