addAudioFilter() {
    const { conf, audios, duration } = this;
    if (isEmpty(audios)) return;

    const normalizeAudio = conf.getVal('normalizeAudio');
    const filters = [];
    const length = audios.length;
    let outputs = '';

    forEach(audios, (audio, index) => {
      const output = `audio${index}`;
      const audioCommand = audio.toFilterCommand({ index, duration });
      filters.push(audioCommand);
      outputs += `[${output}]`;
    });

    if (normalizeAudio) {
      filters.push(`${outputs}amix=inputs=${length}:normalize=0`);
    } else {
      filters.push(`${outputs}amix=${length}`);
    }

    this.command.complexFilter(filters);
  }
Add one or more background sounds
##Note: Usage of adelay/volume filter
[1]adelay=5000|5000,volume=1[a];
[2]adelay=10000|10000,volume=10[b];
[3]adelay=15000|15000,volume=2[c];
[a][b][c]amix=3[a]
@param {array} audios - background sounds
@private