We use Premiere Pro 1.5 with the Matrox RT.X100. Our experiences may be different, but this is what we have encountered:
Premiere creates “Conformed Audio Files”. If you have your scratch disks set to a drive that does not have adequate space to hold these files, Premiere will simply stop the conforming process without notification. When this happens, you will not be able to hear any/some audio.
I would first do a search in Windows for “Conformed”, and delete any files related to the project you are referring to.
Next, start a NEW Premiere project and set your scratch disks (Edit-Preferences-Scratch Disks) to a drive that has plenty of space. Conformed Audio Files can take up a huge amount of disk space. I just deleted a folder with 47 GIGs worth of Conformed Audio Files. That is not typical. Usually for us they are around 1-10 GIGs