It's been so long since I came up against this problem, that I may not be 100% right about this - but have you tried plugging the little audio cable into the back of your Plextor drive? One end goes into the little audio jack, and the other end should plug into your soundcard.
You'll have to open up your PC to check/do this. Hope it helps!
Edit: I don't have said cable plugged into either of my drives, so I tested to see if the sound cames through my headphones. Lo and behold, it didn't...so check for the little audio cable.