• Are you sure the outputs in Digital Performer's Tracks window are set to the pair of outputs that you're trying to listen through (for example Analog 1-2 on a 2408, or phones on an 828mkII)? Are these outputs in italics? If so, you need to make sure your interface is being seen in the Configure Hardware Driver window, then reassign the outputs by choosing "New stereo bundle" in the output column of the Tracks window.
• What about voices? Are all tracks set to a unique voice, or if voices are set to automatic are there enough in the studio size window for all the tracks you have?
• Maybe the tracks are muted in the Mixing Board? Or is Solo mode engaged?
• Is the Master Fader and main outs of your audio interface or mixer turned up?
• Speakers on?
• Everything plugged in the way it should be? Did you try different cables?

Most likely one of the things above is the cause!