When you say convert to audio, does that mean you can put real tracks to it? But that will screw up the melody. won't it?
No, it means that whatever you have in your MIDI file will be turned into an audio track, i.e. no realtracks. Realtracks are audio recordings of professional players, thus they can not follow MIDI.
I respectfully disagree with my friend Herb. If you have a say BiaB piano track that sounds great as is then yes Herb's method is OK. But a dull BiaB track converted to an audio track it will be a dull audio track. For example you have a sax track that is dull, that is no pitch bend, volume changes, or vibrato your sax audio track will be dull, regardless of the price of the MIDI sound source. But you can bring that MIDI track to life with the addition of subtle pitch bends, volume changes and vibrato. Then you convert that live sax track to a live audio track.
The quality of the MIDI sound source reflects mostly on the cost of the MIDI sound source. The expection is that there are some very good free MIDI sounds available BUT I would not encourage you go start getting better MIDI at this point in time.
I will warn you that the BiaB GM MIDI sounds are some of the poorest sounds are when compared to more expensive sounds. PGMusic suggest the free Cakewalk TTS-1 MIDI GM sounds and they are much better. Also some GM sounds may not accept some CCs so don't get frustrated. If you have problems feel free to contact me.
I hope this helps.