How do you decide what voices to assign to the parts of a style that have been converted from a midi file. The drum and bass tracks are obvious but what about the other accompaniment tracks?

Hello Musical Memories:

At the beginning of a good midi file, you have the Patch/sound assigned along with the controllers, like Volume=7, Pan=10, Expression=11, Reverb=91, Chorus=93, Modulation=1 etc....
Each part has a track assigned, and depending on what keyboard you have, that varies, they only track that is always 10 is the Drums, the usual channels to use are from 9 to 16 or 10-16.
So, if you have in your midi other channels, you have to check in the owners manual of your keyboard brand, what are the proper channels to use for each part, as an example, Bass go to channel #X...etc., Chord 1 goes to....etc.
So, to re-cap, you must edit the midi with the sounds of your choice, and also to re-assign the proper channel.

I hope this answers your question.