tween.js

  function tween(start, end, fn) {
    var duration = end - start
      ;

    fn = fn || function (v) { return v; };

    return function (continuation, now) {
      var relativeNow = now - start
        , normalNow
        , res
        ;

      normalNow = duration === 0 ? -1 : relativeNow / duration;

      if (normalNow < 0) {
        res = 0;
      } else if (normalNow > 1) {
        res = 1;
      } else {
        res = fn(normalNow);
      }

      continuation(res);
    };
  }

usage.js

// 30s to 60s
var intermission = tween(30, 60);

function fadeColor(evt) {
  var time = evt.target.currentTime
    , startingSaturation = 80
    ;

  intermission(function (value) {
    var s = startingSaturation + value;
    document.body.style.background = 'hsl(15, ' + s + '%, 40%)';
  }, time);
}

document.getElementsByTagName('audio')
        .addEventListener('timeupdate', fadeColor, false);