This was marked as fixed in the patch logs leading up to v1.2 in this thread: http://forum.baldursgate.com/discussion/21975/update-list
Specifically, the thread lists:
2216: Combat music no longer cuts out when switching between interface screens
Well, It still does cut out after patch 1.2 when opening/closing the inventory screen in some areas.
Steps to replicate:
1. Start a new game with Abdel.
2. C:MoveToArea("AR0503") (Durlag's Tower upper 2nd floor)
3. Draw out ghasts from the north-western room to initiate combat.
4. Pause the game.
5. Wait 5-6 sec to let music flow (I'm not sure if this is required, but this is how I did it).
6. Open inventory screen.
7. Close inventory screen.
--> Observe that the combat music has stopped and (after a moment's delay) started over.
Expected behavior: switching between interface screens when paused in combat does not interrupt combat music.
This is one fix I was really looking forward to, especially in BG2EE and its ToB portion, where you have that wonderful soundtrack and those long and tactically difficult battles in which you often have to micromanage your characters and their inventory.