Thanks for this extremely intriguing idea to use emoji to depict a talking person visually.
You know, some animated films get it right. Sometimes I clearly see utterances like «Thank you!», «Okay», but more often not. Especially older animations don't care and just let the character move the mouth in very simplistic ways: «Babbabbabaa Baababba ba baaa.»
Similarly for that emoji. It is moving the mouth in quite arbitrary ways. It looks like: «Sobbabbabee be <grin> seebee <grin> <frown> babbaa».
To solve this problem we need about twelve emoji for utterances: «ah», «ay», «ee», «oh», «oo», «b», «d», «f», «l», «n», «r» and «s». The «r» emoji must be animated (so we see the trilling tongue). The other sounds are either invisible like «k» or cover several different sounds like «b» also covering «m» and «p».
«Okay» would be rendered by: «oh», «oo», «oo» (standing for «k» wich is invisible) «ay» and «ee».
Edit: Add «th». This is an extremely simple sound to lipread. I remember my delight when I learnt English in my youth and in an movie suddenly realised that an actor said «Thank you».