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Now I bring my midi file back into BIAB and I run it through a synth that actually sounds like a real instrument (how to I get this real sound - I don't want it to sound like a tin can with great articulations). How to make it sound like a real (bass, sax, nylon string guitar, piano, flute, organ, vibes).
So simple! Can this be done? How? How much money roughly?
It's not so simple. BiaB is locked into General MIDI (GM) and it really has to be in order for everyone to be able to hear the same thing, i.e. GM is very easy to use.
You can get some realistic sounds using GM and the SFZ sounds that are included with BiaB. It takes a knowledge of the instrument you are trying to emulate and the MIDI skill via CCs to pull it off. (Note if you take a stock BiaB MIDI track and use a very expensive sound source it still be will static sounding; you need to animate it.) You can get a more realistic bass and nylon guitar by add some vibrato and slides. The piano and organ can sound realistic with the proper playing techniques.
Depending on what GM or SFZ sound source you have other instruments can sound better also. Note that soundfonts and SFZ instruments have many different sounds of the same instrument available. Think like what trumpet sound do you want? Miles? Maynard Ferguson? Satchmo? Sax is one that you will need to spend some money on to get a good sound, but google/bing to see is some free ones will be OK.
Why doesn't PGM automatically do this for you? I surmise it is because every system is different. They know that when one really gets into MIDI one of the first things they do is to drop GM. Lets say you want to have a trumpet slur from a half tone up to the desired tone. My trumpet sound my be set for a half tone slur while your may be set for a two tone slur. Thus mine will sound OK and yours awful! There are no standard for things like that and there souldn't be. It is up to you to set up your instruments.
If you really want to get into better sounding MIDI it does cost. I would suggest you get either Kontakt or SampleTank, both around $400. (I chose Kontakt because of the number of third party sounds for it.) I would wait for a sale on either of these excellent programs. I know that Kontakt goes on sale occasionally for around half price.
BUT before I would spend that kind of money I would start learning MIDI and playing with the sounds you already have. I'm sure others will have suggestions also.
I hope this helps and good luck.