const command = ffmpeg()
const commandArray = []
command.addInput(`./best.mp4`)
command.addInput(`./best2.wav`)
commandArray.push(`[1]volume=0.1[a1]`)
command.addInput(`./voiceover.mp3`)
commandArray.push(`[2]volume=0.9[a2]`)
let ffmpegKeys = '[a1][a2]amix=inputs=2[a]'

commandArray.push(ffmpegKeys)
command.complexFilter(commandArray)
command.addOptions(['-map 0:v', '-map [a]', '-c:v copy'])
.format('mp4')
.on('error', (error) => {
  console.log(error)
})
.on('end', function() {
  console.log('Merging finished !');
})
.save('./merged.mp4');