Record/Playback from external audio source

I have an old microcassette recorder that I have connected to my IMAC via 3.5mm cable - I'm trying to digitize some old recordings. I can't seem to figure out how to get any sound to come out of the Mac. Any thoughts? I had it set up fine on my old PC .

I assume you are trying capture the recordings to your iMac. Make sure that you have the 3.5 cable connected to the line-in port. And make sure that line-in is selected for input in the sound preferences.