I agree with the others. Some more examples: Mother rhymes with other, brother. Mutter rhymes with butter. In some dialects/accents they might be pronounced the same, but not usually.

Mostly the consonants. Mother has the TH sound and Mutter the TT. The vowels are slightly different as well but quite similar.

There’s a difference in pronunciation between them. mother_ /'mʌðər/ mutter_ /ˈmʌtər/