as a customer who purchased Party Hard 2 on GOG i'm puzzled by this description on the store page too...
The way it's worded would imply the OST is bundled with the game as goodies or whatever... But i can tell you that in my library, i only have a game installer and patch(es)
The so called "27 tracks OST" is a feature within the game itself, but not of the "product" you purchase, if i can explain it that way :(
Dunno if it answers your question, but so far that's the factual truth.