audiobuffer-mix.js

function mix(streams) {
	const length = Math.max(...streams.map(s => s.length));
	const numberOfChannels = Math.max(...streams.map(s => s.numberOfChannels));
	const sampleRate = streams[0].sampleRate;
	// assume all streams have the same sample rate for now

	const result = new AudioBuffer({
		length,
		numberOfChannels,
		sampleRate
	});

	streams.forEach(stream => {
		for(let channel = 0; channel < stream.numberOfChannels; channel++) {
			const buffer = result.getChannelData(channel);
			const data = stream.getChannelData(channel);

			for(let i = 0; i < data.length; i++) {
				buffer[i] += data[i];
			}
		}
	});

	return result;
}