Not all computers come with AV inputs in the back. None of mine have. Most computers do have a microphone port. I've been able to record from my keyboard through the microphone port. Either that or you can buy a new sound card with the stereo inputs you need. The third option is to simply work in MIDI, and use midi cables. Almost forgot, recording the first two ways will produce a WAV file. The last I looked there were many freeware wav to mp3 converters.