V stall warning at high altitude on the Airbus
Many thanks to you all for your inputs.
I have now had the opportunity to further check the high altitude approach to stall indications on different A320 simulators and have used 2 x ADRs off (rather than FACs). The result is the same: we go around 15 knots into Vsw before the audio "Stall, stall" alert. The audio seems to coincide with reaching the actual stall AoA, so I think the answer to my original question is that given by Des Dimona: the Vsw black/red band is Mach-based, while the audio is AoA based. The red/black band is therefore not as accurate a reflection of the imminence of the staff as is the audio.
Thanks to everyone who contributed thoughtful and interesting answers.