The achievements are unlocked live when the conditions are met. But only when you finally quit the game the savegame file is written (including your current level) and after that synchronized with the Steam cloud. So it's possible that something happened after you unlocked that achievement; did you remember if the game terminated normally? Or perhaps the savegame has been out of sync next time?
I'm using OpenAL for sound. I output that error only when OpenAL can't find usable audio devices. Perhaps I made a mistake somewhere but at the moment I know of no other computer (from many hundreds) where this error occurs. However I'll keep this in mind.
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