I guess you installed UT Announcer and old 0.9.13 gun sounds?
Banks Loader can be tricky with too many sound mods (especially those that depend on audio_mods.xml)
But banks loader improved recently, so should be easy enough to fix this popup.
Anyway inside "...mods/1.1.0" you should now have two KK91_oldGunSounds_v1.1 files, if you started you game at least once.
Original KK91_oldGunSounds_v1.1.wotmod and one created by banksloader as "compatible" KK91_oldGunSounds_v1.1_BanksLoader_ing.wotmod
To avoid this popup you can:
A) Safely delete original KK91_oldGunSounds_v1.1.wotmod inside "...mods/1.1.0"
B) Delete both KK91_oldGunSounds_v1.1 and KK91_oldGunSounds_v1.1_BanksLoader_ing inside "...mods/1.1.0" BUT first extract wpn_old.bnk and wpn_old.xml from KK91_oldGunSounds_v1.1_BanksLoader_ing.wotmod (using 7zip, Winrar, Winzip...)
and put them inside "...res_mods/1.1.0/audioww"