Here's a recent thread which raises a few possibilities. If you're playing the game in a web browser then you can check for browser options which might auto-mute Flash applications
(also: if you're playing the game in a web browser then please use the Flash Player Projector instead).
Try to isolate the problem. Verify that other applications (such as iTunes) emit sound normally. Try to run other Flash games to check whether it's a SDT-specific thing or a problem with Flash in general. Ensure that the in-game Audio options (checkboxes) are enabled, and that the in-game Volume slider isn't set to 0. Try to install a new copy of
SDT (and/or the
Loader) into a new folder, and then check whether the new copy suffers the same audio problem.