I live in Wisconsin and used two trailer loads of mineral wool. On all sides, including against concrete foundation walls.
So long as you didn't add another vapor retarder in the assembly you should be fine. Once the walls get deep enough the vapor drive is typically to the inside, year round. So the damp proofing and even the concrete act as a vapor retarder against what is trying to come in. I've seen what happens when a 6mil-4mil poly sheet is placed on the basement side of studs...rot.
As for what effect it has...Big (as usual) speaks the truth. I have double studs, sealed every nook and crack, floating ceiling AND loaded with mineral wool. Heavy LFE still rumbles through. 105db - 110db inside the theater, with double doors closed no dialog or midranges gets out, just the 40hz and lower stuff (estimate).
So the mineral wool, I believe, helps dampen what gets past the drywall. So yeah, It helps. But like a lot of "sound isolating" products = a lot of hype/marketing overselling the benefit.