Since the singular is דוֹב, is there a rule bout the change to דוּבִּים?
In most two-consonant nouns and adjectives, where the second consonant is ב, כ or פ, it becomes "hard" (kh -> k, v -> b, f -> p) in plural and feminine (that is, where a vowel is added after). Among those, I think it's the rule that if the singular had /o/ sound it changes to /u/.
But, there are quite a few nouns that satisfy these criteria and don't have these changes (חוף - חופים /hof/ - /hofim/). You can't tell by the singular form which of the two kinds it is, you have to memorize )-:
The correct pronunciation is eh-FOH, but colloquially it's usually pronounced as both EH-foh or EI-foh.