var vid = document.querySelector( "video" );
var audioCtx = new AudioContext();
var source = audioCtx.createMediaElementSource( vid );
var compressor = audioCtx.createDynamicsCompressor();
var now = audioCtx.currentTime;

compressor.threshold.setValueAtTime( -50, now );
compressor.knee.setValueAtTime( 40, now );
compressor.ratio.setValueAtTime( 32, now );
compressor.attack.setValueAtTime( 0, now );
compressor.release.setValueAtTime( .25, now );

source.connect( compressor );
compressor.connect( audioCtx.destination );