Can't you just route the audio through software rather than making a cable connection? In Cubase VST, for example, you can send audio back to the interface using the ASIO driver or through the computer using the built in soundcard. On a Mac, you also need to make a change in the computer's sound control panel. I'm not sure how PC's handle this, but I assume its similar. That way you can avoid the trip to radio shack for 1/8" to 1/4" adapters. You can also save some money and, hopefully, start saving for some studio monitors rather than PC speakers.