US-1200 won't record unless I'm in stereo mix

[Annoyed Bass Player]

I just got a very nice US-1200 interface the other day and I'm trying to set it up with Sonar... it only works for recording however when I have it set to stereo mix on the audio performance tab in the mixer menu. This is fine if I'm just recording without any backing tracks but obviously anything else playing at the same time will get recorded as well and I'm trying to record bass for a few tracks. I even tried to set it up in audacity and even though I have the interface selected as the recording device I don't get anything unless I'm using stereo mix! Can anyone help? I'm sure since I'm a total noob it's probably something very basic that I'm missing.