Combat music does not start in dialogue
Morrowind features numerous quests in which NPCs can become hostile to you in dialogue. All topics besides "Goodbye" will become greyed out, and in vanilla, this is when the exploration music ends and the combat music begins. In OpenMW the music will not transition until you have exited the dialogue window completely.
This does not apply to NPCs you've taunted into becoming hostile - in that case, the behavior is the same as in OpenMW. So it's not fully consistent in the vanilla game.
Tested with Foryn Gilnith in Seyda Neen (quest: "Death of a Taxman") and Fjol outside Hla Oad (quest: "Fjol the Outlaw")
(RM-3664 from redmine: created on 2016-12-15 by August Avatar, , closed on 2017-11-29 by Andrei Kortunov)