no audio while recording in audacity/garage band

[moneyfuxxx]

i'm having a little trouble recording in garage band/audacity. using 10.6.2. anyway, i'm able to record tracks through the line in, but i hear nothing as i am recording even though my monitor is on. after it is recorded i can play it. another problem is once i have the instrumental in garage band/audacity and i create another track for vocals, as soon as i start recording i can't hear the instrumental through my headphones. i checked the inputs/outputs in system preferences and everything looks right in there. so i'm not sure what it could be.