Improve videos with bad audio (too quiet or loud, one channel stereo, ..)

mono_gain_compress_audio.js

(function() {
    let ctx = new AudioContext(); 
    let gain = ctx.createGain();
    let cmpr = ctx.createDynamicsCompressor();
    let src = ctx.createMediaElementSource(document.getElementsByTagName("video")[0]);

    ctx.destination.channelCount = 1;

    let ts = ctx.currentTime;
    gain.gain.setValueAtTime(6, ts);

    cmpr.threshold.setValueAtTime(-50, ts);
    cmpr.knee.setValueAtTime(40, ts);
    cmpr.ratio.setValueAtTime(12, ts);
    cmpr.attack.setValueAtTime(0, ts);
    cmpr.release.setValueAtTime(0.25, ts);

    src.connect(gain);
    gain.connect(cmpr);
    cmpr.connect(ctx.destination);
})();

mono_gain_compress_audio_bookmarklet.js

`javascript:(function()%7Blet%20a%3Dnew%20AudioContext()%3Blet%20p%3Da%5B'createGain'%5D()%3Blet%20i%3Da%5B'createDynamicsCompressor'%5D()%3Blet%20o%3Da%5B'createMediaElementSource'%5D(document%5B'getElementsByTagName'%5D('video')%5B0x0%5D)%3Blet%20h%3Da%5B'currentTime'%5D%3Bp%5B'gain'%5D%5B'setValueAtTime'%5D(0x6%2Ch)%3Bi%5B'threshold'%5D%5B'setValueAtTime'%5D(-0x32%2Ch)%3Bi%5B'knee'%5D%5B'setValueAtTime'%5D(0x28%2Ch)%3Bi%5B'ratio'%5D%5B'setValueAtTime'%5D(0xc%2Ch)%3Bi%5B'attack'%5D%5B'setValueAtTime'%5D(0x0%2Ch)%3Bi%5B'release'%5D%5B'setValueAtTime'%5D(0.25%2Ch)%3Bo%5B'connect'%5D(p)%3Bp%5B'connect'%5D(i)%3Bi%5B'connect'%5D(a%5B'destination'%5D)%3B%7D)()%3B`