Hi nirib,
RiffWorks requires either an ASIO or a CoreAudio compatible interface. The built in soundcard on a pc usually doesn't have an ASIO driver. So, as a result, you won't be able to get any sound.
There's something that some people use, but it's not officially supported by Sonoma Wire Works. It's called ASIO4ALL. It's free. And it turns your built-in soundcard into an ASIO compatible device. However, it doesn't always work.
You're more than welcome to try it out and see if that helps you though.
http://www.asio4all.com/Best of luck.
If you're really thinking about buying RiffWorks though, I would recommend an audio interface. The Line 6 interfaces are great. (Toneport, Guitarport, PODxt). The Stealthplug is really good from IK Multimedia. Both of those get you some really nice amp modeling sounds to play with your electric guitar.