Brian,
This is most strange in that it works when you save using the alternative options (e.g. the WAV tool bar option) but not the Audio menu options.
Just to doublecheck...
Prior to following the below instructions, save your present options using Menu\Options\Save_Preferences. This makes it easy to return to your current set up if needed.
Now, please navigate to the link below and set up the audio and midi exactly as shown. This is the most fundamental setup up for BIAB and it works in 99% of situations.
http://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=461558#Post4615581. Once you've set everything as shown, exit BIAB and restart it.
2. Open one of your songs (maybe a different song from the one you've been trying).
3. Generate it
4. Try rendering the audio.
Has that helped?
One of the quirks of BIAB is the audio rendering requires a suitable VSTi/DXi to be loaded.
Regards,
Noel