Actually, the mac chimes and boots (using an external speaker and hard drive). So this board has no audio and no video + unregulated power supply.... Great!

At this point, I think I'll take a look at the video section (the flyback is new).

As for the sound, I don't think there is much on the AB, the only thing that could fail is the speaker (I can steal one from a dead Mac Classic or a dead SE/30)

EDIT: actually, maybe "no video" is caused by the fact that there is not enough voltage for the oscillator to start, so no voltage coming from out of the flyback

EDIT 2: Yup!, I should have +30V on C14, but I only have about +5V