M is not a tooth. The 'mmm' sound is unrelated to teeth, and used to be an icon for water. A closed-lips sound doesn't signify teeth.
The 'tooth' pictograph looks like a rotated M: Σ which is the Greek sigma, or before that the "sh=šin" (I love hačeks). So "S" actually is "teeth".
Could you post the url link where you are getting all this from? I tried to search for it, but couldn't find it.