I'm assuming your interface is USB. Whenever you're dealing with digital recording, a USB cable counts as an in and out. So you have to make sure Logic isn't using your interface as the output. Go into your mixer (shortcut x) and look right above where it says audio 1, 2, 3, Etc. The output should be set to stereo out. I believe there is also an option in Preferences somewhere. It's the audio tab and it should say what output Logic is using overall, It probably says M-Audio. You want it to say Pro Tools Aggregate I/O...I haven't used Logic in about 1 year so I don't remember that well. I hope this helps.