Turntables -> Mixer -> Garageband -> Stereo

Hi all. New Macbook Pro owner. New to this forum. Sorry if I am repeating earlier posts, but I have been searching far and wide for some kind of solution to the problem I am having.

My turntables are hooked up to my Stanton M.303 3-Channel Mixer, which then has an output that feeds into the Line In on my laptop. In Garageband, I have created a new track using the option for "real insturment" and i can see the volume lines on the track bouncing but I cannot hear any sound. If I record this track, it registers an audio recording - still no sound. When I stop recording and play the track i just recorded, I can hear what I recorded very clearly.

What I want to know is: Why can't I hear my turntables in any situation other than when I play back a recording? I have spent a lot of time messing with the System Preferences and have tried every single combination even aside from the combination that I know should be correct.

Shouldn't I be able to hear what is coming into my Line In even though I am not recording it???? Or when I am recording it???

Any help is greatly appreciated.

thanks for the quick response Mydriasis.

as far as i can remember, i turned the monitor ON last night at one point, and suddenly I could hear through my headphones.... ELATION!!!

But.... then i realized I was just listening to the laptop's built-in microphone.

However, because I was messing with the system preferences so much, i may have momentarily had them set to the built-in mic instead of the Line In at the moment I turned the Monitor to ON.

I will try that again tonight with the monitor on and the Preferences set to Line In. That's what you suggest, right?

Again: thanks for the quick response.