This dream may be about a fear of looking less capable than you'd like at work: you keep going and hide the problem rather than stop. Your front teeth crumbling mid-presentation carries the dream most clearly (front teeth are the ones everyone sees when you speak, so losing them can stand for losing face).
2.1What the images may point to
Covering your mouth may point to the effort of keeping a flaw out of sight while you go on performing. Your manager smiling in the front row may stand for the person whose view of your work counts most, and a smile that doesn't fit what's happening may echo a worry that you can't tell how you're really being seen.
2.2How the pieces fit
Something falls apart in the most visible place, at the most visible moment, and the response is to hide it and carry on.
2.3Another way to read it
Teeth can also stand for not being able to say what you mean. If the hardest part was the talking itself, the dream may lean that way instead.
2.4A question for you
What at work feels like it's crumbling while you keep talking?