polly_bufers.js

const someReallyUsefullFunction = async (text) => {
    // Break in parts small enough to be handle by Polly API
    const parts = divideTextEnoughToBeHandleByPolly(text)
    
    // Concat AudioStreams in one Buffer
    const audioStreams = audios.map(a => a.AudioStream)
    const buffer = Buffer.concat(audioStreams, audioStreams.reduce((len, a) => len + a.length, 0))
    
    ...
}

// Here i am using SSML, but you can use plainText
const generatePollyAudio = (text, voiceId) => {
  const params = {
    Text: `<speak><prosody rate="100%">${text}</prosody></speak>`,
    VoiceId: voiceId,
    TextType: 'ssml'
  }

  return polly.synthesizeSpeech(params).promise().then( audio => {
    if (audio.AudioStream instanceof Buffer) return audio
    else throw 'AudioStream is not a Buffer.'
  })

}