Are you using OS9 or OSX ?
If you are using os 9, check the "monitors and sound" controlpanel and check what audio input you are using.
In digital performer you have to assign what audioinouts you are using or if the tracks are midi, mac av or any other interface.
(I've used DP 3(?) for mac os 8 several years ago, so i have forgotten most of it.)
If you need more help you will need to give us more info.