Electronicly speaking... an audible "buzzer" is not at all hard to design... (the design of the "electronic switch" is very easy)

The only real problem I could see is, every plane will have different stall conditions, depending on airfoil design. You could easily get an estimated area that you would have the stagnant air (as the airfoil sees it at slower speeds). The problem sits at where the calibration of the "switch" is. The "tab" that resides on the wing also needs to be small, but have an area that will give you accurate activation of stall audible sound.

I don't see where a "one fits all" type of device could be manufactured without having some way of getting an accurate speed of the particular aircraft to have the device installed in. So, I would think that the individual would have to design it them selves, and just keep doing trial an error to find that "magic" spot right before stall.

Now, if there is a device out there (I've never seen one, and might be a seriously great idea to get a group of people together and design one... I design electronics for a living, so I'll help if there is interest) that you can mount to your aircraft to give you vectors on the performance of the aircraft in real time, showing actual speed, and g-force that could be plugged into your USB port of your computer, and show you a performance graph of the flight.

next post, I'll post the idea I'm thinking of. (kinda like an accelerator G-meter that they install in indy cars)

In my opinion, a stall alert device is a really cool idea...

Danoman...