Hello, I have a macbook 6.1 which unfortunately have 1 line for input and output at the same time. The problem is that I want to connect my electric guitar, but after I do that I'm limited to the internal speaker since I have no other way to plug other speakers with the guitar using the line in/out. Is there any cheap way to add another audio jack? I have seen some usb to audio jack cables around, but I have no idea if they are gonna work since there is no any software mentioned with them, and it seems short of impossible for the system to just recognize this cable as line in/out. Is an external audio card my only choice? Thanks in advance.