I agree saukko, they need to be told that they must not make noise every time they think, and that verbal diarrhoea is anti social.
Play some games:
Use an egg timer for silence. Award them with golden stars when they're silent for 5 minutes. When they get 10 stars they can go to the playground etc.
Ask them to listen to others for 10 minutes and ask them to count how many people had something to say if they're not talking 19 to the dozen.
Use a wooden spoon/ stick to "allow" different people to speak. They're not allowed to make any noise if they don't have the talking stick.
Ask them to reflect back what you're saying to them, this way they start to actively listen.
"Fine" them for overuse of the "mum, mum mum mum mum" tap. Take stars from their charts if the overuse the mum word.
Seriously, OP you have a duty to get them to stop making "mouth noise" with love, when they are still relatively young,interested in your opinions and compliant, because they will be ignored and avoided, or told harshly and without any thought for their feelings when they're older by strangers. We all know and hate the adult who never stops droning on. We never, ever invite them anywhere fun, and escape from them ASAP.
Believe me to when I say that annoying as it undeniably is for you, who have had these children, and know and love them, it's really, really annoying for those of us who don't know them, and who don't give a monkey's about your kids and their little foibles.
Maybe start socialising them now, before some scary, witch-librarian like me does it for you 