I have a 13" MacBook pro, which has only one 3.5mm jack. I've set it as Sound Input in the System Preferences -> Sound dialog
This way I can record directly from my piano audio output. The problem is, I'm not able to hear anything anymore. The piano has only one 3.5mm jack, and if I put anything in there, it's speakers get turned off.
I'd like to make my MacBook replay the sound it's getting on the sound input in it's built in speakers, so that I can hear what I'm actually playing.
When I try to play any sound directly (like in iTunes), it uses the speakers just fine. But it doesn't play the sound that is being recorded from the 3.5mm input.
Basically this would be the equivalent of hearing yourself speak when talking on Skype.