Would it be possible to build a purely mechanical device that is capable of generating sounds of alphabet either by compression or blowing?
No electronic or electrical parts purely mechanical.
We are talking about an artificial device outside a living system, would be excellent tool to educate illiterate people in poor countries. one alphabet shaped devices made of recyclable material making a single sound
also would be ecofriendly.
We humans do it all the time - I think perhaps the most efficient way to learn others is to actually show them, the "old school" way ;-)
However your question about a mechanical device reminds me of a robot speech project at Kagawa University:
While this project involves pneumatics and electronics, I believe it should be possible (although technically challenging) to construct it purely mechanical with levers, strings, instruction rolls like on self-playing pianos etc.
But in the context of your idea, a "tool to educate illiterate people in poor countries", such a complicated and in the end expensive device is probably a no go. I would go with concept of a good human teacher, perhaps recorded on a device driven by batteries, rechargeable in sun or by mechanical recharging.
