It's a crap shoot, really.
Some soundtracks that w\ere midi-based (especially older PC games), have wound up having better midi instruments used for their OST (like the Doom OST releases), while others have the exact same instruments, but they can sound clearer thanks to the composers having better recording equipment to capture the sound chip's output (like some of Tecno Soft's and Konami's releases). Of course these days, since so much music already is CD quality, an OST can have absolutely no changes done over what you hear in the game. So it can differ from album to album.
That said, a general rule of thumb is that if it's an official/original sound track release, you're going to be getting what you heard in the game. It might sound a bit clearer, or a little better, but it'll basically be what you heard overall.