If you have a decent sound card on your computer you should be able to "record whatever you can hear" from the sound card. I have a Sound Blaster Live Drive card and it comes with a midi synthesizer and a number of different Sound Fonts to create different midi instruments on the synthesizer. So if I wanted to make a MP3 recording, I would first create a Wave (.WAV) recording by selecting the "record whatever you hear" feature in the Sound Blaster Mixer, and it would record whatever sounds the Midi Synthesizer is playing, plus I could also simultaneously mix in 2 audio tracks (vocals, guitar, etc). Then finally you would need to get a software conversion utility to create the .MP3 file from the .WAV file. It's not hard to find such converting software for a low price. Sounds like you want to create a finished recording by using midi synthesizers. If so, try to find some copies of Computer Music magazine, they frequently deal with that topic and suggest hardware for that purpose.