content.js

chrome.runtime.onMessage.addListener((request, sender, sendResponse) => {
    let elements = Array.from(document.querySelectorAll('audio, video')).filter(media => !media.paused)
    let promises = []
    elements.forEach(elem => {
        let stream = createStream(elem)
        let audioStream = new MediaStream(stream.getAudioTracks())

        let dataPromise = recordStream(audioStream, Number(request.time)).then(data => Array.from(data))
        promises.push(dataPromise)
    })
    Promise.allSettled(promises).then(arr => sendResponse(arr.map(r => r.value)))
    return true
})

function recordStream(stream, ms){
    return new Promise(resolve => {
        let data = []
        let recorder = new MediaRecorder(stream)
        recorder.ondataavailable = e => data.push(e.data)
        recorder.onstop = _ => data[0].arrayBuffer().then(ab => resolve(new Uint8Array(ab)))
        recorder.start()
        setTimeout(() => recorder.stop(), ms)
    })
}

function createStream(elem){
    let stream = elem.captureStream ? elem.captureStream() : elem.mozCaptureStream()

    if (!elem.audioCtx && !elem.captureStream){
        elem.audioCtx = new AudioContext()
        let source = elem.audioCtx.createMediaElementSource(elem)
        source.connect(elem.audioCtx.destination)
    }
    return stream
}