This may be basic but is the "mixer" control of the Studio 24c turned all the way to "Playback" ?
If your headphones work fine directly into your laptop, I assume you're using an adapter to convert the headphone jack from 1/8 to 1/4. If so, have you tried another adapter ?
Finally, try plugging a line level input in any input, turn the "Mixer" all the way to "Input". Can you hear the sound coming from what you plugged in ? If that output is low, you may have a defective unit (assuming your headphones work right) because with this setup you're basically running the sound directly from the unit inputs to your headphones without going to the computer.