Monitoring via the PCI-324 Sound Manager Driver through an application like Peak takes place at the hardware level. This is set in Cue Mix. Cue Mix handles the routing of the audio signal for the 2408. Here you can turn channels on and choose output assignments for these channels. Cue Mix's Master fader must also be turned on; this controls the overall Cue Mix level.

As an example, the Monitor button in Peak's record window only toggles the state of monitoring within Peak. However, you still need to properly setup Cue Mix to monitor inputs through outputs on the 2408. Refer to your Cue Mix documentation on how this is done.