Once you've installed a UIRES nationality package with all the screens and attendant voice packs and update your country.cfg file accordingly, you have to modify the allegiance values in the MISC DATA sections in the dp's of all your nationality aircraft to reflect their specific nationalities. For example, all British aircraft should have an allegiance designation of 1:
[MISC_DATA]
unit_family=1
category=1
allegiance=1
max_group_size=12
min_speed=240
cruise_speed=525
max_speed=787
min_alt=50
cruise_alt=7519
max_alt=20000
entered_service=9/1/40
crew=1
This designation is what the sim uses to select the voice files, among other things, to use in combat sessions, because it corresponds with the data structure of your country.cfg file. If the dp allegiance and country.cfg agree, but the addon UIRES screens and voice packs are not present in your sim for your non-U.S. allied nationality, then the sim will default to the primary allied nationality -- U.S. -- upon which the sim was built.