TinyChan

Not bytebeat, but here's how I'd play notes in JavaScript…

var audioCtx=new(window.AudioContext||window.webkitAudioContext)();
let semitones={C:-9,D:-7,E:-5,F:-4,G:-2,A:0,B:2};
function noteToFreq(note){
    //Notes can either be integers or strings.
    let semitone;
    if(typeof note==="number"){semitone=note;}
    else{
        let match=note.match(/^([A-G])([b#]?)(-?\d+)$/i);
        if(!match)throw new Error("Invalid note: "+note);
        semitone=semitones[match[1].toUpperCase()];
        let accidental=match[2],octave=parseInt(match[3],10);
        if(accidental==="#")semitone++;
        if(accidental==="b")semitone--;
        semitone+=(octave-4)*12; //4 is the octave of A4
    }
    return 440*Math.pow(2,semitone/12);
}
function playNote(note,duration,velocity,start){
    let osc=audioCtx.createOscillator(),
        gain=audioCtx.createGain(),
        freq=noteToFreq(note);
    osc.frequency.setValueAtTime(freq,audioCtx.currentTime+start);
    gain.gain.setValueAtTime(velocity,audioCtx.currentTime+start);
    osc.connect(gain);
    gain.connect(audioCtx.destination);
    osc.start(audioCtx.currentTime+start);
    osc.stop(audioCtx.currentTime+start+duration);
}
function scheduleNotes(notes){
    notes=notes.slice().sort((a,b)=>a.start-b.start);
    let startTime=audioCtx.currentTime,
        index=0,
        interval=setInterval(()=>{
        let currentTime=audioCtx.currentTime-startTime;
        while(index<notes.length&&notes[index].start<=currentTime*1000){
            let{note,duration,velocity,start}=notes[index];
            playNote(note,duration,velocity,start);
            index++;
        }
        if(index>=notes.length)
            clearInterval(interval); //Stop
    },50);
}
//scheduleNotes([{note:"C4",start:0,duration:10,velocity:.5},{note:"A4",start:5,duration:5,velocity:1}]);
scheduleNotes((()=>{let a=[],n=[..."CDEFGA"];for(let i=0;i<100;i++){let note=n[Math.floor(Math.random()*n.length)]+(4+Math.floor(Math.random()*3)),duration=.1+Math.random()*.2,velocity=Math.random()*.25,start=i/10;a.push({note,duration,velocity,start});}return a;})());