Most of the used voice samples in BGS are from milinks Customsounds.oxp from 2006. If I recall it right milinks recorded his wife instead of using speech synthesis. Some other samples are made by aegidian (dunno which voice was used), but some years ago I've asked him if he still has the raw data. Unfortunately not anymore. All files have been modified for BGS (normalized, filtered, etc).
For some later additions, like the countdown, I've used Marytts with the Prudence voice + Protools and Audacity. Protools is Mac-only though, but some years ago digidesign (the former owner) released free versions for Windows as well.
Hope it helps a bit and have fun .-)