Cubase 5 / MoPho connection

I’ve been using Cubase 5 for a while but never with MIDI. I just picked up a MoPho desktop and am trying to incorporate some of its sounds into my pieces. I can drive it from a Cubase track (using a USB/MIDI cable to go from my computer’s USB port to MoPho’s MIDI in), and I can plug my headphones into the MoPho’s audio out and hear it fine. However, I want to hear (and record) that sound along with my other tracks in Cubase. Therefore, along with the previous setup, I connected MoPho’s audio out to my soundcard (using audio phone jacks). I can then set up a Cubase audio track, with “stereo in” as the only avaialble input bus, but it doesn’t detect anything. I can set up a Cubase MIDI track and monitor the input with no problem, which frankly confuses me a little bit since I thought it was audio, not MIDI. Any suggestions?