Specifically, I want to digitize a lot of old audio tapes. Can I just play them on a good deck and feed the signal into the audio input of my iMac (assuming I can adapt the output plug to fit the input jack)? If the sound goes through the system, I have Audio Hijack Pro to record it. I know that iMic is a way to do it, but I'm not sure I can find the one I used to have! 
Thanks for advice.
Thanks for advice.