Mund is masculine, but "sein" declines like "ein" (so-called mixed declination). You don't say "einer Apfel"! ein Mund - sein Mund
seiner is a word, but it is used when there is no noun after it. (so it's not a determiner, but a pronoun then)
Sein Mund ist seiner - His mouth is his
(well, in English it's better to see in "her":)
Ihr Mund ist ihrer - Her mouth is hers
In fact they are quite different, because they differ in two aspects.
The vowel in "Mund" is an "u" (like "oo" in English"), but it is short (a short "oo" does not exist in English), whereas the vowel in "Mond" is an "o", and it is long. A long "o" of this kind does not exist in English, it is somewhat close to the "o" in "bone".
The two words do not differ in their end consonant. "d" and "t" are both pronounced as a "t" in this position.