Hi Pumps,
To appreciate BIAB's limitations with midi-to-audio, it's necessary to understand the differences between the two. (Please forgive me if you know this. I just thought that it's probably better to start at the beginning and then you can select the bits that are relevant.)
MIDI: These are a set of instructions, usually in text, that tell a synthesiser (software or hardware) to play a particular pitch note. The instructions also give information on sysnthesiser sound to use, note-length, volume, how the note begins, what happens to the sound as it's played, how the note ends. These instructions are created when a midi track is generated in BIAB.
The bottom line is that midi is not a sound; just instructions to make a sound with a proper interface.
AUDIO: is a sound file. One of the most common ways of obtaining a music audio is to record someone playing an instrument. This is what Realtracks are. When you look at a Realtrack Guitar that is accredited to Brent Mason (for example), this means that Brent has sat in a studio and played his guitar while PG Music recorded his playing. PGM technology then uses electronics to take this audio, adjust its tempo, change its pitch and ultimately patch it together into a coherent musical track (this is what happens when the sound is generated). At all times though, the audio file stays as an audio file.
Midi can easily be converted to audio because all it requires is to record the midi track as it plays a synthesiser.
To convert audio to midi, though, it's more complicated and it's necessary to have software that can interpret pitch correctly and then create a set of midi instructions.
Realcharts are an attempt to notate what a musician played when s/he created a Realtrack. These are mostly done after the playing has happened and often by a person who wasn't involved with the playing or recording. These Realcharts are stored in BIAB as midi notation. Sometimes the Realcharts are highly accurate and sometimes they are an excellent approximation of a performance. Realtracks and Realcharts are two different things. They work in partnership. When RTs are generated, parallel RCs are also generated.
Hope this helps,
Noel