I'm afraid I don't have anything more than a hypothetical answer here; assuming their lips can touch, it's possible for them to make labial sounds.  As an experiment, try sticking your index fingers beneath your lower lip about where their teeth would be, and try talking.  It's difficult!  But, still possible.  Unfortunately the dialogue recorded in-game doesn't reflect how this would sound.

I too have noticed this. It's quite bizarre but if there is no way to find it in other places there probably is'nt a single way to speak like an Orsimer. Seeing how the dialogue does'nt reflect this nobody thought people would try to imitate the speech.