The spelling of "march up cannon mouth" may seem confusing at first due to its unique pronunciation. The correct pronunciation of this phrase is /mɑːrtʃ ʌp kænən maʊθ/. The "ch" is pronounced as a "k" sound, the "r" is silent, and the "a" in "canon" is pronounced like "ah". The word "mouth" also has a distinct pronunciation. By breaking down the sounds and understanding the International Phonetic Alphabet (IPA) transcription, the spelling of "march up cannon mouth" becomes clearer.