Let's say you want to multiply all values within an existing AudioBuffer by 2. You could do that by looping over all values to apply the computation.

for (let channel = 0; channel < audioBuffer.numberOfChannels; channel += 1) {
    const channelData = audioBuffer.getChannelData(channel);

    for (let sample = 0; sample < channelData.length; sample += 1) {
        channelData[sample] *= 2;
    }
}

This will edit all values in place. If you want to copy all values to a new AudioBuffer instead you can do it as follows.

const copiedAudioBuffer = new AudioBuffer({
    length: audioBuffer.length,
    numberOfChannels: audioBuffer.numberOfChannels,
    sampleRate: audioBuffer.sampleRate
});

for (let channel = 0; channel < audioBuffer.numberOfChannels; channel += 1) {
    const channelData = audioBuffer.getChannelData(channel);
    const copiedChannelData = copiedAudioBuffer.getChannelData(channel);

    for (let sample = 0; sample < channelData.length; sample += 1) {
        copiedChannelData[sample] = channelData[sample] * 2;
    }
}

The example above creates an AudioBuffer with the exact same properties as the existing one. It will leave the existing AudioBuffer untouched.