HTML5 <audio> playback with fade in and fade out

The jQuery way...

$audio.animate({volume: newVolume}, 1000);

Edit: where $audio is a jQuery-wrapped audio element and newVolume is a double between 0 and 1.

Edit: The element's effective media volume is volume, interpreted relative to the range 0.0 to 1.0, with 0.0 being silent, and 1.0 being the loudest setting, values in between increasing in loudness. The range need not be linear. http://www.w3.org/html/wg/drafts/html/master/embedded-content.html#effective-media-volume

Edit: People are posting vanilla JavaScript implementations, so I'll post a vanilla TypeScript one that preserves the jQuery swing animation (just strip out the type info if you want to run this in JavaScript). Disclaimer, this is completely untested:

export async function adjustVolume(
    element: HTMLMediaElement,
    newVolume: number,
    {
        duration = 1000,
        easing = swing,
        interval = 13,
    }: {
        duration?: number,
        easing?: typeof swing,
        interval?: number,
    } = {},
): Promise<void> {
    const originalVolume = element.volume;
    const delta = newVolume - originalVolume;

    if (!delta || !duration || !easing || !interval) {
        element.volume = newVolume;
        return Promise.resolve();
    }

    const ticks = Math.floor(duration / interval);
    let tick = 1;

    return new Promise(resolve => {
        const timer = setInterval(() => {
            element.volume = originalVolume + (
                easing(tick / ticks) * delta
            );

            if (++tick === ticks + 1) {
                clearInterval(timer);
                resolve();
            }
        }, interval);
    });
}

export function swing(p: number) {
    return 0.5 - Math.cos(p * Math.PI) / 2;
}

Old question but if anyone is looking for a vanilla JS way to do this I just wrote something up for project and thought I'd post it here since my search for a solution was in vain. If you are already working with a video or audio element, there's a good chance you don't really need to use jQuery to control the object anyways.

function getSoundAndFadeAudio (audiosnippetId) {

    var sound = document.getElementById(audiosnippetId);

    // Set the point in playback that fadeout begins. This is for a 2 second fade out.
    var fadePoint = sound.duration - 2; 

    var fadeAudio = setInterval(function () {

        // Only fade if past the fade out point or not at zero already
        if ((sound.currentTime >= fadePoint) && (sound.volume != 0.0)) {
            sound.volume -= 0.1;
        }
        // When volume at zero stop all the intervalling
        if (sound.volume === 0.0) {
            clearInterval(fadeAudio);
        }
    }, 200);

}

This version doesn't allow for editing the fadeout time (set to 2 seconds) but you could pretty easily argumentize it. To fully generisize this, extra logic would be needed to also first check what the volume was set to in order to know the factor by which to fade it out. In our case, we preset the volume to 1 already and browser volume control is out of the users hands as it's for a slideshow thing so it wasn't needed.

To get to a specific part of the audio you'd want to check the seekable timerange and just set the currentTime randomly based on what's available.

I created a simple lib for this using setTimeout().

Source code is available here:

https://github.com/miohtama/Krusovice/blob/master/src/tools/fade.js

Here are a couple of functions to fade-out current song and fade-in new song.

HTML:

<audio loop autoplay="1" onPlay="audioVolumeIn(this);">
         <source src="/test/new/imgs/audio_bg.ogg" type="audio/ogg; codecs=vorbis">
         <source src="/test/new/imgs/audio_bg.wav" type="audio/wav">
         <source src="/test/new/imgs/audio_bg.mp3" type="audio/mpeg">
</audio>

Javascript:

    function audioVolumeIn(q){
       if(q.volume){
          var InT = 0;
          var setVolume = 0.2; // Target volume level for new song
          var speed = 0.005; // Rate of increase
          q.volume = InT;
          var eAudio = setInterval(function(){
              InT += speed;
              q.volume = InT.toFixed(1);
              if(InT.toFixed(1) >= setVolume){
                 clearInterval(eAudio);
                 //alert('clearInterval eAudio'+ InT.toFixed(1));
              };
          },50);
       };
   };
   
   function audioVolumeOut(q){
       if(q.volume){
          var InT = 0.4;
          var setVolume = 0;  // Target volume level for old song 
          var speed = 0.005;  // Rate of volume decrease
          q.volume = InT;
          var fAudio = setInterval(function(){
              InT -= speed;
              q.volume = InT.toFixed(1);
              if(InT.toFixed(1) <= setVolume){
                 clearInterval(fAudio);
                 //alert('clearInterval fAudio'+ InT.toFixed(1));
              };
          },50);
       };
   };