A lot of people have asked me how to record keyboards and other instruments/devices into ACID. This is my official response.
Summary: An explanation as to which cables you need in order to record sound from an external device (keyboard, rack, turntable, mic, etc.) into Sony ACID (or any other software multitrack) and how to connect these cables to your computer's stock sound card.
If you are not using a stock soundcard, i.e. the sound card that your computer came with, this video may not help you.