monophthongize
/ˈmɒnə(f)θɒŋʌɪz/(also monophthongise)
verb
1 [with object] To change or convert (a vowel sound) into a monophthong.
2 [no object] To change or convert a vowel sound to a monophthong.
Origin
Early 20th century; earliest use found in Modern Language Notes. From monophthong + -ize, after diphthongize.