John,
how can that be possible for Blu-ray? I too have DVD A Studio b128, VPro 10 and 11 (b425) and DVD A Pro b124 on the same computer and I don’t find a way to set AC3 in File/New nor in File/Disc Properties.
Also from DVD A Studio Help:
Audio format
Select the control, click the button, and choose a setting to indicate the format whether you want to use PCM Stereo audio, AC-3 Stereo audio, or AC-3 5.1-surround audio.
Only PCM Stereo audio is available for Blu-ray Disc projects.