I had a similar discussion about this with a friend. I'm a massive Witcher 3 fan and he played through God of Wars and said the soundtrack was incredible and better than Witcher's. I was too stubborn to accept that and was trying to think of ways to prove him wrong. I eventually came to the conclusion that God of Wars WAS better but I still didnt like it anywhere as much as I did Witcher. I came to the realization that the experience I had playing Witcher 3 significantly improved the experience I have when I listen to the soundtrack by itself thus significantly increasing the quality of the soundtrack for myself. I believe the answer boils down to not only the quality of the standalone soundtrack but also the experience you had playing the game that the soundtrack originates from.