Why is there no audio for midi keyboard even with assigned instrument?

Okay, so I have my akai midi keyboard coected correctly and assigned to an instrument in Studio One 3 AND have master volume as well as track volume tued up but still have no audio. Last night I had the same problem and DUH it was because I had all audio ruing through my headphones in my audiobox. This time, the audiobox is not interfering and I don't know what the problem could be. Please help!!