The reason for it ,
because a Ches at the end of a word that has a patach under it is called. a patach genuva.
To pronounce a patach genuva you add a last syllable " ah" (or sefardim "wah")
Hence, Noach Koach Tapuach etc.
The same rule (Patach Genuvah) applies to a word that ends with a Hei and has a dogesh (dot)
Many people pronounce incorrectly. in Hallel ,Milifnei ELOHA Yaakov or. in Al Cheit, ELOHA Selichos
The correct pronounciation is ELOAH (Ah
not Ha )
BMG SHOULD CHANGE THEIR STATIONARY
My nephew R. P. Perl Shlita was meorer that Beth Medrash GovoHA. is misspelled *.
Proper spelling would be GovoAH .
Govoah for Zochor is spelled gimmel veis VOV HEI (Patach Genuva - AH )
Gevoha for Nekeivah is spelled gimmel veis HEI HEI (Reg Patach under first Hei - HA )
Sukkah (nekeiva) shehi Gevoha (2 Heis reg Patach)
Bais Midrash (zochor) Govoah (Patach Genuvah)
* only english spelling is incorrect - hebrew spelling is correct gimmel veis vov hei (Patach Genuva)