Respect the bgm vol setting for at3, mp3, video

Volume was basically ignored after f311901e5eb4af957d6c6b3e8962aa0bdad8e7ea, so that setting no longer did anything.

This applies it to mp3, atrac3/+, and video.  Of course, music through sas (FF Tactics), won't have this setting, and I think at3+ sound effects through sas will have both volumes applied.  But that was the case before anyway, I think?

And obviously, music/sound effects manually decoded (LittleBigPlanet) won't have either setting applied.

-[Unknown]