D F Tweedie
This is because of the latency ... or delay time ... in most audio drivers. Unless you are using ASIO drivers, there will be a significant delay between when you hit 'play' and when the audio actually starts appearing (odd choice of word!) out of your speakers due to the filling up of the audio buffers prior to the processing of the digital audio bits..
Later when you play the recorded audio back, it is then out of sync with the rest of the audio. There may be a way in BIAB to compensate/ adjust something, but I don't know.
A simple solution ... if you're have Real Band or some other audio program, is to render all the BIAB song to audio, transfer this and your vocal performance into separate tracks and then 'slide' one track until it matches the other.
You can can also download a free program called ASIO4All (just google it) which will give you a choice of using an ASIO driver for your audio in BIAB. Some folks have excellent results with ASIO drivers, which generally come with the purchase of audio interface hardware as specifically designed for that equipment. Some, myself included, have had no love trying to use my ASIO drivers in BIAB. I would be near suicidal were it not for the fact that I have other audio programs that love my ASIO drivers and I can simply export my BIAB song as a midi file and use it in my regular sequencer.
[Non-text portions of this message have been removed]