Just to be clear, there are two kinds of dropdown menus in the editor : the classic HTML ones for simple values (like the one for voice) and the ones with embedded preview (character profiles, sprites, music, sounds, etc.)
The issue you are mentioning affects all menus of the second kind I guess ?
Those were quite complex to implement and make an intensive use of JS event handling, so I'm not really surprised that IE would have trouble with those : IE has never been great at following the standards with event-related stuff... But anyway, I guess we should try to find out what goes wrong here - the issue being that I'm not planning to upgrade to W10 before a while so I can't really reproduce.
If knowledge can create problems, it is not through ignorance that we can solve them. Si le savoir peut créer des problèmes, ce n'est pas l'ignorance qui les résoudra.
( Isaac Asimov )