The spelling of the phrase "most mouthing" may seem a little odd at first glance. However, it can be explained using IPA phonetic transcription. The "s" in "most" is pronounced as a voiceless alveolar fricative /s/. The "th" in "mouthing" is voiced, pronounced as a voiced dental fricative /ð/. The final "ing" sound is pronounced as a velar nasal /ŋ/. So, the spelling of "most mouthing" accurately reflects the pronunciation of each individual sound in the phrase.