The different spellings are, of course, all English transliterations, but the vowel sounds are different in different dialects of Arabic. The name written in Arabic is is transliterated as MHMD or MHMT. Very few sounds in Arabic are congruent with sounds in English or other civilized languages so the D and T represent a sound somewhere in that range. The written name does not change but different dialects provide different vowels so long as the consonants stay the same.
It all sounds like “Dirka Dirka Dirka Muhammad Jihad” to me.