An audio interface likely won't make a significant difference to your sound quality, and won't make the type of difference that I think you're talking about. An audio interface will improve the sound over your Soundblaster, but it won't fundamentally change the sound. What kind of changes do you want - better instrument sounds?

What I think you would want to do, for the MIDI tracks at least, is load up your favourite VST instruments in Band-in-a-Box (or export the MIDI to a DAW for that same purpose).

For the Realtracks, you could use EQ or compressor plugins on them to adjust how they sound, but as they're audio recordings you won't be able to use instrument plugins on them.

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