silence when sent to STP

I sent my audio from my sequence to Sound Track Pro, and whe the audio track is loaded in STP, there is no sound. I can see the audio meter bouncing but there is no sound. And if I toggle back to FCP, I have no sound there either until I quit STP. So, I though maybe they were competing for the sound card and could not run at the same time (although its worked just fine before), but even if I quit FCP, I have no sound in STP. what could possibly be wrong??