If you look at the network requests when the page loads you can see a good hint that something is going on, 'dialuptone.mp3' is requested from the server (and there are DOM <audio> nodes to play it). I guess if they had wanted to be sneaky they would have generated the sound with the Web Audio API [1], but I'm not sure what browser support is like for that yet.

[1] http://webaudio.github.io/web-audio-api/