Should be easy, and your card includes all the software you'd need to do it, although other stuff will do it better.
First off, if the audio is coming in through the line-in on the soundcard, all you should need to do is open up Creative Mediasource, and record the audio from the line-in. If you're using some other way of inputting the sound, simply change the source method, though "What U Hear" should be used only as a last resort, as it in my opinion creates a lower quality recording. Now, the software itself lets you record as an mp3, so you could do that, or record as a wave and use an mp3 compression program such as Creative Audio Converter, or LAME if you want good quality, to change the waves into mp3.