Difficult to say. While playing or reading a good soundtrack surely helps the immersion, and it helps make memorable scenes even more memorable. But that's the thing, it mostly just enhances what the game is already doing through its story/characters/setting. If those fail a good soundtrack wouldn't be capable of salvaging it.

But what a soundtrack does do is heavily influence a game's lasting impact for me. I tend to take a good long look at the soundtrack of anything I just finished and then start listening to a select few tracks on a regular basis. The better the soundtrack, the more I immerse myself right back in it, the more of a lasting impact the whole thing leaves. Even now I still listen to the Umineko soundtrack on an almost daily basis, and that's surely a factor in how I came to love it as much as I do.

So I guess I could say, a soundtrack can't make a game good, but it can make it great.