There is a software version of the Rolad SoundCanvas MIDI syntheser out there. You can search for it on the web- one place that has it is http://www.pgmusic.com/rolandvs.htm for $30.
That's a piece of software that'll do all your MIDI in the computer and then output it to your audio-only soundcard. That's the way you're going to have to go if you can't add something like an SBLive to your system.