Right, the EQ controls the volume of each of the 10 (each one corresponds to a slider) sounds. (Every sound has been previously decomposed into its frequency components.) The Web Audio API is used for browsers that have it present, else HTML5 audio elements are used. The slider just adjusts the volume of the sound (exponentially of course!), leading to an equalizer effect with no filtering required. (Garnered from looking at the code.)
Interestingly, this could be done using one file (rather than ten) using a Web Audio API filter node. This would limit the site to only Web Audio API compatible browsers. Unfortunately, at this time, that's only newer versions of Chrome, Safari, and I believe FF nightly (slight API variations). However there is some mobile support through recent Chrome and Safari implementations.