What you can do is first go to Multitrack mode and right click on the side (where it has track 1, track 2, and so forth) and note what you have listed under Playback Device. And like PapillonIrl said, go to Options>Settings>Devices and make sure that it lists the same soundcard under Waveform Playback that it does under Playback Device in Multitrack Mode.
If that doesn't work, there might be some configuring you need to do with your soundcard, so if that doesn't work just tell us what soundcard you have and we'll do the best to help you out.