HTML5 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;
}