If 衔 means 'hold in the mouth', why bother adding '在嘴里'?
What is being held? Majority of Chinese VP especially less common ones are disyllabic or as in the present case monosyllabic verb combined with adverbial preposition phrase (介词短语),if object makes clear that mouth is involved there is no need to mention it, e.g. (bkrs examples) 燕子衔泥│他衔着一个大烟斗,衔环相报(wounded bird holds ring),