What happens if you inhale oxygen then enter a room filled with helium and utter a sentence exhaling co2 into the helium?

Answer 1

Nothing remotely exciting. The pitch of your voice will be normal because there is no helium passing your vocal cords. CO2 is exhaled in every breath and will be mixed with the helium as it would be in other atmospheres.

Only if you remain in the room and continue to breath the helium will there be any ill effects. Helium isn't poisonous but the lack of oxygen would cause asphyxiation and eventually death.