In the word וַיִּתְמַהְמָהּ in Genesis 19:16, does the first ה end a syllable so the shva under it is a shva nach and consequently the ה itself is also silent, so the word is pronounced vayitmamahhhhh, or does it begin the fourth syllable and the shva is a shva na, so the word is pronounced vayitmahemahhhhh?
Both letters ה in that word are pronounced the same. They both end syllables beginning with מ.
Shva nach on consonants at the end of a word is conventionally omitted from writing, so in the relatively unusual case where a letter which usually just marks a vowel is intended as a consonant, a dot is included in that letter to indicate its 'harder' status. Only a letter without any vowels on it is entirely silent since it serves itself as a vowel for the previous consonant.