I have a late-2008 iMac. I have been using my FirstAct electric guitar via my computer's line-in port with a 1/4-to-1/8 adapter just fine. Suddenly, it is not working. It used to be associated with an aggregate device, but I have deleted it to no avail. The guitar's volume is turned up 100%, input volume is normal, but no sound is being picked up. In the audio midi setup window, the "M" channel volume slider is grayed out and turned all the way down, but I don't know if that's normal or not. Please help!