Hi Ian,
You say you use
Guitar Pro... Have you tried GP5's RSE? I think it produces really decent sound...
But, as scotty_b said, you'll hardly have a sound you could enjoy in, like - you hit play and then sit back and enjoy in your symphony orchestra playing... MIDI is not about playing, it's about writing down notes... MIDI files are keeping the notes... Then when you hit play they give those notes to MIDI mapper, which has sound samples for those notes... The quality of those samples (i.e. the quality of MIDI mapper, i.e. the quality of your sound card) determines what will you hear as a result...
So, you could 1) get a better sound card (at least that's what I heard of, never had one so I don't know how great would be the benefit of it), or if you use GP 2) get RSE whose sound libraries are pretty decent, or 3) just understand that MIDI files are not audio files, and that they should be used as a transitional solution, a good tool so you can hear (not just read) what had been writen down (i.e. they have a whole different purpose)...
If you can get that RSE, you can almost always import MIDI files into GP and see the standard notation, guitar tabs and hear them...
btw. here's the quote from wiki: "MIDI does not transmit audio—it simply transmits real time digital data providing information such as the type and intensity of the musical notes and technical cues played during a performance."
Hope this helps a bit...
All best
edit: and 737blues was a little faster on this RSE idea...
