Sometimes an entrance can be partly or completely obscured by a wall. Check the auto map, to see if you can see some blue sticking out somewhere. Running around methodically searching with the mouse can also help.
Occasionally the entrance can be a pile of rocks, rather than the usual fountain thingy.
Rarely part of a level can be separated from the rest (which sounds like the case here), which could contain the entrance to the next level.
In the main game there is a way to get a summoning doll through a wall and then enter and leave the BF to get your characters there (
armory trick in act 1). Since you are in the BF already, that will not help (it could in acts 2 or 3, where you would have teleporter stones and could have a doll drop one on the other side of the wall to get your characters past).
Alt-tabbing out of the game and back can shift character positions slightly, so if you get both characters as close as you can to a wall you may be able to alt-tab repeatedly to get them through. Doing this with other NPCs around can cause problems if they get through a wall and become inaccessible.
Teleporting anywhere (via hex editor)Alternately, you can just skip the rest of that dungeon. You may miss one or more BF quest solutions, but completing the BF dungeons is not necessary.
The dungeons are generated the first time you enter the BF for that act. In order to get a different dungeon layout, you would have to reload a save before you first entered the BF.
I never noticed a problem with music. It might not change immediately when changing locations (usually at the end of the current piece), but I didn't have the same song played repeatedly.
