Silent Piano Roll

I have a wierd problem. I imported a series of midi tracks, and opened up HALion, setting the 'ouput' of the tracks to HALion, which plays back the midi files fine. However, when I then edit it Piano Roll, I cannot hear the sounds I have loaded up in HALion when I press one of the piano keys in the pian roll, alhough I can hear any alterations I make in Piano Roll when i play back.

Any ideas?

HI

Yes - the Audio Engine is running (checked that one!) and it is the same with other plug ins - but only with imported midi - not with stuff I write in Sonar itself.