This is how I got it to work in case anyone else has the same issue:

let mp3Data = [];
let audioContext = new (window.AudioContext || window.webkitAudioContext)();

let audioBuffer = await getPCM();
let samples = getSamples(audioBuffer);
let audioBlob = getBlobMp3(samples);


// pcm: pulse-code modulation, used in audio
async function getPCM() {
  // chunks from mediaRecorder
  let blob = new Blob(chunks, { type: "audio/webm" });
  let arrayBuffer = await blob.arrayBuffer();

  return await audioContext.decodeAudioData(arrayBuffer);
}

function getSamples(audioBuffer) {
  let channelData = audioBuffer.getChannelData(0); // Mono channel (0 for left)

  return float32To16BitPCM(channelData);
}

function float32To16BitPCM(float32Array) {
  let int16Array = new Int16Array(float32Array.length);

  for (let i = 0; i < float32Array.length; i++) {
    int16Array[i] = Math.max(-1, Math.min(1, float32Array[i])) * 0x7fff; // 0x7FFF = 32767
  }

  return int16Array;
}

function getBlobMp3(samples) {
  let mp3encoder = new lamejs.Mp3Encoder(
    1, // Mono channel
    44100, // 44.1kHz
    128, // 128 kbps for MP3 encoding
  );

  let sampleBlockSize = 1152; // multiple of 576

  for (let i = 0; i < samples.length; i += sampleBlockSize) {
    let sampleChunk = samples.subarray(i, i + sampleBlockSize);
    let bufferMp3 = mp3encoder.encodeBuffer(sampleChunk);
    if (bufferMp3.length > 0) {
      mp3Data.push(bufferMp3);
    }
  }

  let finalBufferMp3 = mp3encoder.flush();
  if (finalBufferMp3.length > 0) {
    mp3Data.push(finalBufferMp3);
  }

  return new Blob(mp3Data, { type: "audio/mp3" });
}

I had to:

• get an array buffer from a blob made of audio chunks from mediarecorder
• get an audiobuffer from that array buffer using audioContext
• convert that audiobuffer from 32bit to 16bit
• pass the 16bit array to lamejs to convert to mp3
• make another blob from the mp3
• pass up the audio blob to the server to be processed.

I am not sure if it is the correct answer but it did work for me. If anyone knows a better way to do this, please comment/answer.