"Mussen" is spelled with a double "s" in the middle, as it is derived from the German verb "müssen", which means "to have to" or "must." In IPA phonetic transcription, the word is pronounced as /ˈmʊsən/. The sound /s/ is represented by the letter "s", and the letter "u" represents the vowel sound /ʊ/. The double "s" in the middle represents the sound /s/ followed by a short pronunciation of the vowel sound /ə/.