Reproduce mono buffers in all channels if a stereo buffer is present

[Alynva]

Currently, mergeAudio and concatAudio takes the maximum number of channels as the number of channels in the output buffer but for each input buffer it only gets its own channels and apply them to the corresponding channel in the output buffer. Its somewhat like this:

in: [[M], [L, R]]
out: [M+L, R]

The first buffer (which was mono) isn't reproduced in the right channel in the output buffer.

Mono buffers should be reproduced in all output channels.

[vanceingalls]

i had the same issue and ended up converting mono buffers to stereo before merging

import Crunker from "crunker";

export const convertMonoToStereo = (audioBuffer: AudioBuffer) => {
  const crunker = new Crunker();
  if (audioBuffer.numberOfChannels === 1) {
    // Duplicate the single channel to create a stereo buffer
    const stereoBuffer = crunker.context.createBuffer(
      2,
      audioBuffer.length,
      audioBuffer.sampleRate
    );

    // Normalize the audio since duplicating the channel increases the signal strength
    const normalizationFactor = 1 / Math.sqrt(2);
    stereoBuffer
      .getChannelData(0)
      .set(
        audioBuffer
          .getChannelData(0)
          .map((sample) => sample * normalizationFactor)
      );
    stereoBuffer
      .getChannelData(1)
      .set(
        audioBuffer
          .getChannelData(0)
          .map((sample) => sample * normalizationFactor)
      );

    return stereoBuffer;
  } else {
    return audioBuffer;
  }
};