Generating sound on the fly with javascript/html5

You can use the Web Audio API in most browsers now (excepting IE and Opera Mini).

Try out this code:

// one context per document
var context = new (window.AudioContext || window.webkitAudioContext)();
var osc = context.createOscillator(); // instantiate an oscillator
osc.type = 'sine'; // this is the default - also square, sawtooth, triangle
osc.frequency.value = 440; // Hz
osc.connect(context.destination); // connect it to the destination
osc.start(); // start the oscillator
osc.stop(context.currentTime + 2); // stop 2 seconds after the current time

If you want the volume lower, you can do something like this:

var context = new webkitAudioContext();
var osc = context.createOscillator();
var vol = context.createGain();

vol.gain.value = 0.1; // from 0 to 1, 1 full volume, 0 is muted
osc.connect(vol); // connect osc to vol
vol.connect(context.destination); // connect vol to context destination
osc.start(context.currentTime + 3); // start it three seconds from now

I got most of this from experimenting in chromium while reading the Web Audio API Working Draft, which I found from @brainjam 's link.

I hope that helps. Lastly, it is very helpful to inspect the various objects in the chrome inspector (ctrl-shift-i).

Sure! You could use the tone synthesizer in this demo:

audioCtx = new(window.AudioContext || window.webkitAudioContext)();

show();

function show() {
  frequency = document.getElementById("fIn").value;
  document.getElementById("fOut").innerHTML = frequency + ' Hz';

  switch (document.getElementById("tIn").value * 1) {
    case 0: type = 'sine'; break;
    case 1: type = 'square'; break;
    case 2: type = 'sawtooth'; break;
    case 3: type = 'triangle'; break;
  }
  document.getElementById("tOut").innerHTML = type;

  volume = document.getElementById("vIn").value / 100;
  document.getElementById("vOut").innerHTML = volume;

  duration = document.getElementById("dIn").value;
  document.getElementById("dOut").innerHTML = duration + ' ms';
}

function beep() {
  var oscillator = audioCtx.createOscillator();
  var gainNode = audioCtx.createGain();

  oscillator.connect(gainNode);
  gainNode.connect(audioCtx.destination);

  gainNode.gain.value = volume;
  oscillator.frequency.value = frequency;
  oscillator.type = type;

  oscillator.start();

  setTimeout(
    function() {
      oscillator.stop();
    },
    duration
  );
};

frequency
<input type="range" id="fIn" min="40" max="6000" oninput="show()" />
<span id="fOut"></span><br>
type
<input type="range" id="tIn" min="0" max="3" oninput="show()" />
<span id="tOut"></span><br>
volume
<input type="range" id="vIn" min="0" max="100" oninput="show()" />
<span id="vOut"></span><br>
duration
<input type="range" id="dIn" min="1" max="5000" oninput="show()" />
<span id="dOut"></span>
<br>
<button onclick='beep();'>Play</button>

Have fun!

I got the solution from Houshalter here: How do I make Javascript beep?

You can clone and tweak the code here: Tone synthesizer demo on JS Bin

Compatible browsers:

• Chrome mobile & desktop
• Firefox mobile & desktop Opera mobile, mini & desktop
• Android browser
• Microsoft Edge browser
• Safari on iPhone or iPad

Not Compatible

• Internet Explorer version 11 (but does work on the Edge browser)

Using the HTML5 audio element

Cross-browser generative sustained audio using JavaScript and the audio element isn't currently possible, as Steven Wittens notes in a blog post on creating a JavaScript synth:

"...there is no way to queue up chunks of synthesized audio for seamless playback".

Using the Web Audio API

The Web Audio API was designed to facilitate JavaScript audio synthesis. The Mozilla Developer Network has a Web Based Tone Generator that works in Firefox 4+ [demo 1]. Add these two lines to that code and you have a working synth with generative sustained audio upon keypress [demo 2 - works in Firefox 4 only, click the 'Results' area first, then press any key]:

window.onkeydown = start;  
window.onkeyup = stop;

The BBC's page on the Web Audio API is worth reviewing too. Unfortunately, support for the Web Audio API doesn't extend to other browsers yet.

Possible workarounds

To create a cross-browser synth at present, you'll likely have to fall back on prerecorded audio by:

1. Using long prerecorded ogg/mp3 sample tones, embedding them in separate audio elements and starting and stopping them upon keypress.
2. Embedding an swf file containing the audio elements and controlling playback via JavaScript. (This appears to be the method that the Google Les Paul Doodle employs.)